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ANDREW 

SMITH 

HALLIDIE: 


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THE  ART 


OF 


ILLUMINATION 


BY 


LOUIS    BELL,    PH.    D. 


OF  THE 


UNIVERSITY 


NEW  YORK 

McGRAW   PUBLISHING  CO. 

114  LIBERTY  STREET 
1902 


HALUDIE 


•  COPYRIGHTED,    1902, 

BY  THE 

MCGRAW    PUBLISHING    COMPANY, 
NEW  YORK. 


PREFACE. 


THIS  volume  is  a  study  of  the  utilization  of  artificial 
light.  It  is  intended  to  deal,  not  with  the  problem  of  dis- 
tributing illuminants,  but  with  their  application,  and  treats 
of  the  illuminants  themselves  only  in  so  far  as  a  knowl- 
edge of  their  peculiarities  is  necessary  to  their  intelligent 
use.  To  compress  the  subject  within  reasonable  bounds, 
it  has  been  necessary  to  discuss  general  principles  rather 
than  concrete  examples  of  artificial  lighting.  The  science 
of  producing  light  changes  rapidly  and  the  apparatus  of 
yesterday  may  be  discarded  to-morrow,  but  the  art  of 
employing  the  materials  at  hand  to  produce  the  required 
results  follows  lines  which  are  to  a  very  considerable 
extent  subject  to  fairly  well-defined  laws.  Sins  against 
these  laws  are  all  too  common,  the  more  so  since  artificial 
light  has  become  relatively  cheap  and  easy  of  applica- 
tion. If  this  outline  of  a  complex  art  shall  tend  to  avert 
even  some  of  the  commoner  errors  and  failures  in  illumi- 
nation, it  will  have  served  its  purpose.  The  author  here 
desires  to  express  his  obligations  to  the  beautiful  treatise 
of  M.  Allemagne  for  illustrations  of  early  fixtures  and  to 
numerous  friends,  notably  Mr.  Luther  Stieringer,  for 
valuable  material  and  suggestions. 

November,   1902. 


1 16753 


CONTENTS. 


CHAPTER  PAGE 

•  I.     LIGHT  AND  THE  EYE,                                   0        .        .        .  i 

•II.     PRINCIPLES  OF  COLOR,  23 

VIII.     REFLECTION  AND  DIFFUSION, 38 

-IV.     THE  MATERIALS  OF  ILLUMINATION — ILLUMIMANTS  OF 

COMBUSTION,            .        . 56 

V.     THE  MATERIALS  OF  ILLUMINATION— INCANDESCENT  BURN- 
ERS,            83 

VI.     THE  ELECTRIC  INCANDESCENT  LAMP,        ....  95 

VII.     THE  ELECTRIC  ARC  LAMP, 140 

•VIII.     SHADES  AND  REFLECTORS, 163 

IX.     DOMESTIC  ILLUMINATION,           ......  183 

X.     LIGHTING  LARGE  INTERIORS 211 

.  XI.     STREET  AND  EXTERIOR  ILLUMINATION,     ....  244 

XII.     DECORATIVE  AND  SCENIC  ILLUMINATION,          .        .        .  275 

XIII.  THE  ILLUMINATION  OF  THE  FUTURE,       .        .        .        .301 

XIV.  STANDARDS  OF  LIGHT  AND  PHOTOMETRY,       0        .        .  313 


IX 


OF  THE 

(    UNIVERSITY;) 

I 


THE  ART  OF  ILLUMINATION. 


CHAPTER  I. 

LIGHT    AND    THE    EYE. 

WHILE  even  the  Esquimaux  and  the  Patagonian  use 
artificial  light  and  all  civilized  peoples  count  it  a  necessity, 
it  is  seldom  used  skillfully  and  with  proper  knowledge  of 
the  principles  that  should  govern  its  employment.  Since 
the  introduction  of  electric  lights  that  very  facility  of  ap- 
plication which  gives  them  unique  value  has  encouraged 
more  zeal  than  discretion  in  their  use.  It  is  the  purpose 
of  the  present  volume  to  set  forth  some  of  the  fundamental 
doctrines,  optical,  physiological,  and  aesthetic,  which 
underlie  the  proper  use  of  artificial  illuminants,  and  to 
point  out  how  they  may  be  advantageously  adapted  to 
existing  conditions. 

To  begin  with,  there  are  two  general  purposes  which 
characterize  two  quite  distinct  branches  of  the  art  of  illu- 
mination. First  comes  the  broad  question  of  supplying 
artificial  light  for  carrying  on  such  avocations  or  amuse- 
ments as  are  extended  into  the  hours  of  darkness.  Quite 
apart  from  this  is  the  case  of  scenic  illumination  directed 
at  special  objects  and  designed  to  produce  particular 
effects  or  illusions.  Lighting  a  shop  or  a  house  exempli- 
fies the  one,  lighting  a  picture  gallery  or  the  stage  of  a 
theater  the  other.  Each  has  a  distinct  purpose,  and  re- 


2  THE  ART   OF   ILLUMINATION. 

quires  special  means  for  its  accomplishment.  Confusing 
the  purposes  or  mixing  the  methods  often  leads  to  serious 
mistakes.  Sometimes  both  general  and  scenic  illumina- 
tion have  to  be  used  coincidently,  but  the  distinction  be- 
tween them  should  be  fully  realized  even  when  it  cannot 
fully  be  preserved. 

General  illumination,  whether  intended  to  serve  the 
ends  of  work  or  play,  must  fulfill  the  following  condi- 
tions: it  must  be  amply  adequate  in  amount,  suitable  in 
kind,  and  must  be  so  applied  as  not  to  react  injuriously 
upon  the  eye. 

i/  It  must  be  remembered  that  the  human  eye  is  not  merely 
a  rather  indifferent  optical  instrument,  but  a  physical 
organ  which  has  through  unfathom- 
able ages  accumulated  the  characters 
wrought  upon  it  by  evolution,  until  it 
bears  the  impress  and  incurs  the  limita- 
tions of  its  environment.  It  works 

Fi     i— Indian       ^>es*  over  a  ratner  limited  retinal  area 
Goggles.  and  through  a   range  in   intensity  of 

light  which,  although  great,  is  yet  immensely  smaller 
than  the  range  available  to  nocturnal  creatures.  It 
has,  moreover,  become  habituated  to,  and  adapted  to, 
light  coming  obliquely  from  above,  and  resents  strong 
illumination,  whether  natural  or  artificial,  from  any 
other  direction.  It  seems  to  be  well  established,  for  ex- 
ample, that  the  distress  caused  by  the  reflected  glare  from 
sand,  or  water,  or  snow,  and  the  grave  results  which  fol- 
low prolonged  exposure  to  it,  are  due  not  so  much  to  the 
intensity  of  the  light  as  to  the  fact  that  it  is  directed  up- 
ward into  the  eye  and  is  quite  insufficiently  stopped  by 
the  rather  transparent  lower  eyelid.  Ordinary  dark 
glasses  are  small  protection  in  this  case,  but  if  the  lower 


LIGHT    AND    THE   EYE.  3 

part  of  the  eye  be  thoroughly  guarded  no  difficulty  is 
found.  The  Alaskan  Indians  have  evolved  a  very  effect- 
ive protection  against  snow  blindness  in  the  shape  of 
leather  goggles  with  the  eye  arranged  as  shown  in  Fig.  i . 
The  eyepiece  is  merely  a  round  bit  of  dark  leather  with  a 
semicircular  cut  made  for  the  peep  hole,  the  resulting  flap 
being  turned  outward  and  downward,  so  that  the  eye  is 
fully  guarded  from  brilliant  upward  beams.  Blackening 
the  whole  lower  eyelid  with  burnt  cork  is  stated  by  one 
distinguished  oculist  to  be  completely  efficacious  for  the 
same  reason. 

It  is  more  than  likely  that  the  bad  effects  ascribed  to  the 
habit  of  reading  while  lying  down  are  due  largely  to  the 
unwonted  direction  of  the  illumination,  as  well  as  to  the 
unusual  direction  of  the  eye's  axis. 

All  these  matters  are  of  fundamental  importance  in 
planning  any  illumination  to  facilitate  hard  visual  work. 
Their  significance  is  that  we  are  not  at  liberty  to  depart 
widely  from  the  distribution  and  character  of  natural  day- 
light illumination.  Of  course,,  one  realizes  immediately 
that  the  eye  is  neither  fitted  nor  habituated  to  working  to 
advantage  in  anything  like  the  full  strength  of  sunlight, 
but  its  more  general  properties — steadiness,  absence  of 
pronounced  color,  downward  cblique  direction,  wide  and 
strong  diffusion,  freedom  from  sharp  and  black  shadows 
— these  must  be  followed  rather  closely  in  ordinary  artifi- 
cial illumination,  or  the  eye,  that  has  been  taking  form 
through  a  million  years  of  sunlight  and  skylight,  will  re- 
sent the  change.  The  eye  is  automatically  adjustable,  it 
is  true,  for  wonderfully  diverse  conditions,  but  persistent 
and  grave  changes  in  environment  are  more  than  it  can 
bear. 

Now  from  a  practical  standpoint  the  key  to  artificial 


4  THE   ART   OF   ILLUMINATION. 

illumination  is  found  in  the  thoughtful  contemplation  of 
what  is  known  as  Fechner's  law,  relating  to  the  sensitive- 
ness of  the  eye  to  visual  impressions.  It  is  stated  by 
Helmholtz  substantially  as  follows :  "  Within  very  wide 
limits  of  brightness,  differences  in  the  strength  of  light 
are  equally  distinct  or  appear  equal  in  sensation,  if  they 
form  an  equal  fraction  of  the  total  quantity  of  light  com- 
pared." That  is,  provided  the  parts  of  the  visual  picture 
remain  of  the  same  relative  brightness  the  distinctness  of 
detail  does  not  vary  materially  with  great  changes  of  ab- 
solute brightness.  Now  since,  barring  binocular  vision, 
our  whole  perception  of  visible  things  depends,  in  the  ab- 
sence of  strong  color  contrasts,  upon  differences  of  illumi- 
nation, the  importance  of  the  law  just  stated  needs  little 
comment.  It  implies  what  experience  proves,  that  within 
a  rather  wide  range  of  absolute  brightness  of  illumination 
our  vision  is  about  equally  effective  for  all  ordinary  pur- 
poses. 

Fechner's  law,  to  be  sure,  fails  when  extremely  brilliant 
lights  are  concerned.  Few  persons  realize,  for  instance, 
that  the  sun  is  twice  as  bright  at  noon  as  it  is  when  still 
10  to  15  degrees  above  the  horizon,  still  less  that  its  bril- 
liancy is  reduced  more  than  a  hundred  fold  as  its  lower 
limb  touches  the  horizon.  Yet  while  the  eye  does  not  de- 
tect very  small  changes  or  properly  evaluate  large  ones  in 
a  body  so  bright  as  the  sun,  the  mere  fact  that  one  can  see 
to  work  or  read  about  equally  well  from  sunrise  to  sunset 
is  most  convincing  as  to  the  general  truth  of  the  law. 
Full  sunlight  at  noon  is  generally  over-bright  for  the  eye 
if  it  falls  directly  upon  the  work,  but  with  half  of  it  one 
can  get  along  very  comfortably. 

All  this  is  most  important  from  the  standpoint  of  arti- 
ficial illumination,  since  it  means  that  within  rather  wide 


LIGHT    AND    THE    EYE.  5 

limits  of  intensity  artificial  lighting  remains  about  equally 
effective  for  most  practical  purposes. 

The  actual  amount  of  illurriination  necessary  and  desir- 
able, the  terms  by  which  we  measure  it,  and  the  laws  that 
govern  its  intensity  are  matters  of  primary  importance 
which  must  now  occupy  our  attention. 

A  simple  and  definite  standard  of  light  is  greatly  to  be 
desired,  but  we  do  not  yet  possess  it.  The  p/actical  and 
generally  legal  standard  in  English-speaking  countries  is 
the  standard  candle.  This  is  defined  to  be  a  spermaceti 
candle  of  certain  definite  dimensions,  weighing  one-sixth 
of  a  pound  avoirdupois,  and  burning  120  grains  per  hour. 
Such  a  candle  is  a  fairly  steady  and  uniform  source  of 
light,  and  although  far  less  precise  than  would  be  desira- 
ble, has  served  a  most  useful  purpose  as  a  standard  of  light. 
From  this  a  standard  of  illumination  is  derived  by  defin- 
ing the  distance  at  which  this  standard  intensity  produces 
a  certain  definite  illumination,  which  forms  an  arbitrary 
unit.  Thus  one  candle-foot  has  come  to  be  a  definite  unit 
of  illumination,  i.  e.,  the  direct  illumination  given  by  a 
standard  candle  one  foot  from  the  object  illuminated.  Of 
course,  it  is  entirely  empirical,  but  it  serves  the  practical 
purpose  of  comparing  and  defining  amounts  of  illumina- 
tion just  as  well  as  if  it  were  a  member  of  the  C.  G.  S. 
system  in  good  and  regular  standing. 

A  unit  of  illumination  frequently  used  abroad  is  the 
bougie-meter,  similarly  derived,  with  the  meter  as  unit  dis- 
tance. This  is  sometimes  known  as  the  lux,  but  un- 
happily there  is  neither  any  convenient  and  practicable 
absolute  standard  of  light  nor  any  definitely  settled  nofnen- 
clature  of  the  subject,  so  that  to  save  confusion  the  writer 
prefers  to  adhere  for  the  present  to  candle-foot,  which  is 
at  least  specific,  and  bears  a  determinable  relation  to  the 


6  THE   ART   OF   ILLUMINATION. 

bougie-meter.  (Approximately  the  candle-foot  equals 
eleven  bougie-meters. ) 

For  any  light  the  illumination  at  one  foot  distance  is 
obviously  a  number  of  candle  feet  numerically  equal  to  the 
candle  power  of  the  light. 

At  distances  other  than  one  foot  the  illuminating  power 
is  determined  by  the  well  defined,  but  often  misapplied, 
"  law  of  inverse  squares."  This  .law  states  that  the  in- 
tensity of  light  from  a  given  source  varies  inversely  as  the 
square  of  the  distance  from  that  source.  Thus  if  we  have 
a  radiant  point  (P,  Fig.  2)  it  will  shine  with  a  certain  in- 
tensity on  a  surface  a  b  c  d  at  a  distance  e  P.  If  we  go  to 
double  the  distance  (E  P)  the  same  light  which  fell  on 
abed  now  falls  on  the  area  A  B  C  D  of  twice  the  linear 
dimensions  and  four  times  the  area,  and  consequently  the 
intensity  is  reduced  to  one-fourth  of  the  original  amount. 
Thus  if  P  be  one  candle  and  e  P  one  foot,  then  the  illumi- 
nation at  e  will  be  one  candle-foot,  and  at  E  one-fourth 
candle-foot. 

This  law  of  inverse  squares  is  broadly  true  of  every  case 
of  the  free  distribution  of  energy  from  a  point  within  a 
homogeneous  medium,  for  reasons  obvious  from  the  in- 
spection of  Fig.  2.  It  does  not  hold  in  considering  a 
radiant  surface  as  a  whole,  nor  for  any  case  in  which  the 
medium  is  not  homogeneous  within  the  radii  considered. 

By  reason  of  these  limitations,  in  problems  of  practical 
illumination  the  law  of  inverse  squares  can  be  considered 
only  as  a  useful  guide,  for  it  is  far  from  infallible,  and 
may  lead  to  grossly  inaccurate  results.  It  is  exact  only  in 
the  rare  case  of  radiation  from  a  minute  point  into  space 
in  which  there  is  no  refraction  or  reflection.  A  room 
with  dead  black  walls,  lighted  by  a  single  candle,  would 
furnish  an  instance  in  which  the  illumination  could  be 


LIGHT   AND    THE   EYE. 


computed  by  the  law  of  inverse  squares  without  an  error 
of-  more  than  say  2  or  3  per  cent.,  while  a  white  and  gold 
room  lighted  by  a  well  shaded  arc  light  would  illustrate 
an  opposite  condition,  in  which  the  law  of  inverse  squares 
alone  would  give  a  result  grossly  in  error. 

Fig.  3  shows  how  completely  deceptive  the  law  of  in- 
verse squares  may  become  in  cases  complicated  by  refrac- 
tion or  reflection.  Here  one  deals  with  an  arc  light  of 
perhaps  10,000  nominal  cp.  as  the  source  of  radiation, 

A 


Fig.  2.— Illustrating  Law  of  Inverse  Squares. 

but  a  very  large  proportion  of  the  total  luminous  energy  is 
concentrated'  by  the  reflector  or  lens  system  into  a  nearly 
parallel  beam  which  maintains  an  extremely  high  lumi- 
nous intensity  at  great  distances  from  the  apparatus.  If 
the  beam  were  actually  of  parallel  rays  its  resultant  illumi- 
nation would  be  uniform  at  all  distances,  save  as  dimin- 
ished by  the  absorption  of  the  atmosphere,  probably  not 
over  iQper  cent,  in  a  mile  in  ordinary  clear  weather,  since 
the  absorption  of  the  entire  thickness  of  the  atmosphere 
for  the  sun's  light  .is  only  about  16  per  cent. 

The  searchlight  furnishes  really  a  special  case  of  scenic 
illumination,  which  frequently  depends  upon  the  use  of 
concentrated  beams  in  one  form  or  another,  so  that  one 
must  realize  that  a  very  considerable  branch  of  the  art  of 
illumination  imposes  conditions  not  reconcilable  with  the 
ordinary  application  of  the  law  of  inverse  squares. 


8  THE   ART   OF   ILLUMINATION. 

It  is  worth  while  thus  to  examine  the  law  in  question, 
because  it  is  a  specially  flagrant  example  of  a  principle 
absolutely  and  mathematically  correct  within  certain  rigid 
limitations,  but  partially  or  wholly  inapplicable  in  many 
important  cases. 

Having  considered  the  unit  strength  of  light  and  the 


Fig.  3- — Beam  From  Searchlight. 

unit  strength  of  illumination,  the  other  fundamental  of 
artificial  lighting  is  the  intensity  of  the  luminous  source — 
generally  known  as  intrinsic  brightness.  Optically  this 
has  no  very  great  or  direct  importance,  but  physiologically 
it  is  of  the  most  serious  significance,  and  perhaps  deserves 
more  thoughtful  attention  than  any  other  factor  in  prac- 
tical illumination.  It  is  of  the  more  consequence,  as  it  is 
the  one  thing  which  generally  receives  scant  considera- 
tion, and  is  left  to  chance  or  convenience. 

By  intrinsic  brightness  is  meant  the  strength  of  light 
per  unit  area  of  light-giving  surface.     If  we  adopt  the 


LIGHT   AND    THE    EYE.  9 

standard  candle  as  the  unit  of  light,  and  adhere  to  English 
measures,  the  logical  unit  of  intrinsic  brightness  is  one 
candle  power  per  square  inch.  One  then  may  conven- 
iently express  the  brightness  of  any  luminous  surface  in 
candle  power  per  square  inch,  and  thus  obtain  a  definite 
basis  of  comparison. 

Although  a  measure  of  intrinsic  brightness  is  obtained 
by  dividing  the  candle-power  of  any  light  by  the  area  of 
the  luminous  surface,  this  latter  quantity  is  very  difficult 
to  determine  accurately,  since  with  the  exception  of  the 
electric  incandescent  filament  no  source  of  light  is  any- 
where nearly  of  uniform  brilliancy  over  its  entire  surface. 
For  the  sake  of  comparison  we  can,  however,  draw  up  an 
approximate  table  by  assuming  equal  brightness  over  the 
generally  effective  lighting  area  of  any  radiant.  It  should 
be  distinctly  understood  that  the  values  tabulated  are  only 
average  values  of  quantities,  some  of  which  are  incapable 
of  exact  determination  and  others  of  which  vary  over  a 

.wide  range  according  to  conditions. 
0  j    ^^ 

INTRINSIC    BRILLIANCIES    IN    CANDLE    POWER    PER 

SQUARE  INCH 
SOURCE.  BRILLIANCY.  NOTES. 


horizon 

Arc  light 10,000  to  100,000         Maximum  about  200,000  in  crater. 

Calcium  light 5,ooo 

Nernst  "  glower" 1,000         Unshaded. 

Incandescent  lamp 200-300         Depending  on  efficiency. 

Melting  platinum. 130         I  sq.  cm.  =  18.5  c.p. 

Enclosed  arc 75-100         Opalescent  inner  globe. 

Acetylene  flame 75-ioo 

Welsbach  light 20  to  25 

Kerosene   light 4  to  8         Very  variable. 

Candle 3  to  4 

Gas  flame 3  to  8        Very  variable. 

Incandescent  (frosted).  ..  2  to  5 

Opal  shaded  lamps,  etc.  .  0.5  to  2 

The  striking  thing  about  this  table  is  the  enormous  dis- 
crepancy between  electric  and  other  lamps  of  incandes- 


io  THE   ART   OF   ILLUMINATION. 

cence  and  flames  of  the  ordinary  character.  The  very 
great  intrinsic  brilliancy  of  the  ordinary  incandescent  lamp 
is  particularly  noteworthy  and,  from  the  oculist's  stand- 
point, menacing. 

Everyone  is  familiar  with  the  distress  caused  the  eye  by 
sudden  alternations  of  light  and  darkness,  as  in  stepping 
from  a  dark  room  into  full  sunlight,  or  even  in  lighting 
the  gas  after  the  eye  has  become  habituated  to  the  dark- 
ness. The  eye  is  provided  with  a  very  wonderful  auto- 
matic "  iris  diaphragm  "  for  its  adjustment  to  various 
degrees  of  illumination,  but  it  is  by  no  means  instanta- 
neous, although  very  prompt,  in  its  action.  Moreover,  the 
eye  after  resting  in  darkness  is  in  an  extremely  sensitive 
and  receptive  state,  and  a  relatively  weak  light  will  then 
produce  very  noticeable  after-images.  These  after- 
images, such  as  are  seen  in  vivid  colors  after  looking  at 
the  sun,  are  due  to  retinal  fatigue. 

If  the  image  of  a  brilliant  light  is  formed  upon  the 
retina,  it  produces  certain  very  considerable  chemical 
changes,  akin  to  those  produced  by  light  upon  sensitized 
paper.  In  so  doing  it  temporarily  exhausts  or  weakens 
the  power  of  the  retina  to  respond  at  that  point  to  further 
visual  impressions,  and  when  the  eye  is  turned  away  the 
image  appears,  momentarily  persistent,  and  then  reversed, 
dark  for  a  white  image,  and  of  the  complementary  hue  for 
a  colored  one.  This  after-image  fades  away  more  or  less 
slowly,  according  to  the  intensity  of  the  original  impres- 
sion, as  the  retina  recovers  its  normal  sensitiveness. 

A  strong  after-image  means  a  serious  local  strain  upon 
the  eye,  and  shifting  the  eye  about  when  brilliant  light  can 
fall  upon  it  implies  just  the  same  kind  of  strain  that  one 
gets  in  going  out  of  a  dark  room  into  bright  sunshine. 
The  results  of  either  may  be  very  serious.  In  one  case 


LIGHT   AND    THE   EYE.  n 

recently  reported  a  strong  side  light  from  an  unshaded  in- 
candescent lamp  set  up  an  inflammation  that  resulted  in 
the  loss  of  an  eye.  The  light  was  two  or  three  feet  from 
the  victim,  whose  work  was  such  that  the  image  of  the 
filament  steadily  fell  on  about  the  same  point  on  the  retina, 
at  which  point  the  resulting  inflammation  had  its  focus. 
A  few  weeks'  exposure  to  these  severe  conditions  did  the 
mischief.  This  is  an  extreme  case,  but  similar  conditions 
may  very  quickly  cause  troubleTj  A  year  or  two  since  the 
writer  was  at  lunch  facing  a  window  through  which  was 
reflected  a  brilliant  beam  from  a  white  painted  sign  in  full1 
sunlight  just  across  the  street.  No  especial  notice  was 
taken  of  this,  until  on  glancing  away  a  strong  after-image 
of  the  sign  appeared,  and  although  the  time  of  exposure 
was  only  ten  or  fifteen  minutes,  the  net  result  was  inability 
to  use  the  eyes  more  than  a  few  minutes  at  a  time  for  a 
fortnight  afterwards. 

To  certain  extent  the  eye  can  protect  itself  from  too 
brilliant  general  illumination  by  closing  up  the  iris,  and  it 
always  does  so,  reducing  the  general  brightness  of  the 
retinal  images,  as  one  regulates  the  illumination  on  a 
photographic  plate.  The  following  results  of  experi- 
ments by  Lambert  will  give  an  idea  of  the  way  in  which 
the  pupil  reacts  to  variations  of  light.  The  radiant  used 
was  a  hole  in  a  shutter  admitting  bright  skylight  to  a 
darkened  room. 

RELATIVE    DISTANCE.  AREA  OF   PUPIL   IN    SQ.    MM. 

1  7-3 

2  13.0 

3  16-6 

4  20.5 

5  25.0 

6  30.6 

7  36.8 

44-5 

9  48.0 

10  57.1 


12  THE  ART   OF   ILLUMINATION. 

But  a  light  of  great  intrinsic  brilliancy  produces  so 
strong  an  image  that  it  may  cause  trouble  even  when 
the  aperture  of  the  eye  is  stopped  to  the  utmost  limit 
provided  by  nature.  In  the  effort  to  accomplish  this 
adjustment  the  iris  closes  so  far,  when  a  brilliant  light  is 
in  the  field  of  vision,  that  the  rest  of  the  field  may  be 
dimmed  so  much  as  to  interfere  with  proper  vision,  quite 
aside  from  any  question  of  fatigue  induced  by  the  bright 
image  wandering  over  the  retina  as  the  eye  is  shifted. 

In  general  terms  the  iris  adjusts  itself  with  reference  to 
the  brightest  light  it  has  to  encounter,  so  that  if  there  is  in 
the  field  of  vision  a  source  of  light  of  great  intrinsic  bril- 
liancy, the  working  illumination  may  be  highly  unsatisfac- 
tory. The  same  principle  coupled  with  retinal  fatigue 
accounts  for  one's  inability  to  see  beyond  a  brilliant  light, 
as  in  driving  towards  an  arc  lamp  hung  low  over  the 
street. 

A  very  simple  experiment,  showing  the  effect  of  a  bril- 
liant source  of  light  on  the  apparent  illumination,  may  be 
tried  as  follows :  Light  a  brilliant  lamp,  unshaded,  in  a 
good-sized  room,  preferably  one  with  darkish  paper. 
Then  put  on  the  light  an  opal  or  similar  shade..  It  will 
be  found  that  the  change  has  considerably  improved  the 
apparent  illumination  of  the  room,  although  it  has  really 
cut  off  a  good  part  of  the  total  light.  Moreover,  at  points 
where  there  remains  a  fair  amount  of  illumination,  the 
shade  has  improved  the  reading  conditions  very  materi- 
ally. If  one  is  reading  where  the  unshaded  light  is  at  or 
within  the  edge  of  the  field  of  vision,  the  improvement 
produced  by  the  shade  is  very  conspicuous.  Lowering" 
the  intrinsic  brilliancy  of  the  light  has  decreased  the  strain 
upon  the  eye  and  given  it  a  better  working  aperture. 

As  a  corollary  to  these  suggestions  on  the  effect  of 


LIGHT   AND    THE   EYE.  13 

bright  lights  on  our  visual  apparatus  should  be  mentioned 
the  fact  that  sudden  variations  in  the  intensity  of  illumi- 
nation seriously  strain  the  eye  both  by  fatigue  of  the 
retina,  due  to  sudden  changes  'from  weak  to  strong  light, 
and  by  keeping  the  eye  constantly  trying  to  adjust  itself 
to  changes  in  light  too  rapid  for  it  properly  to  follow. 

A  flickering  gaslight,  for  example,  or  an  incandescent 
lamp  run  at  very  low  frequency,  strains  the  eye  seriously 
and  is  likely  to  cause  temporary,  even  if  not  permanent, 
injury. 

The  persistence  of  visual  impressions  whereby  the 
retinal  image  remains  steady  for  an  instant  after  the  ob- 
ject ceases  to  affect  the  eye  furnishes  a  certain  amount  of 
protection  in  case  of  very  rapid  changes  of  brilliancy.  It 
acts  like  inertia  in  the  visual  system. 

In  the  case  of  arc  and  incandescent  lamps  the  thermal 
inertia  of  the  filament  or  carbon  rod  also  tends  physically 
to  minimize  the  changes,  but  with  a  low  frequency  alter- 
nating current  they  may  still  be  serious. 

The  exact  frequency  at  which  an  incandescent  lamp  on 
an  alternating  circuit  begins  to  distress  the  eye  by  the  flick- 
ering effect  depends  somewhat  on  the  individual  eye  and 
somewhat  on  the  mass  of  the  filament.  In  general,  a  i6-cp 
lamp  of  the  usual  voltages,  say  100  to  120  volts,  begins  to 
show  flickering  at  or  sometimes  a  little  above  30  cycles  per 
second;  one  foreign  authority  noting  it  even  up  to  40 
cycles.  At  25  cycles  the  flickering  is  very  troublesome  to 
most  eyes,  and  at  20  cycles  or  below  it  is  generally  quite 
intolerable.  In  looking  directly  at  the  lamp  the  filament  is 
so  dazzling  that  the  fluctuations  are  not  always  in  evidence 
at  their  full  value,  and  a  low  frequency  lamp  is  quite  likely 
to  be  the  source  of  trouble  to  the  eye  even  when  at  first 
glance  it  -appears  to  be  quite  steady. 


i4  THE   ART   OF   ILLUMINATION. 

Lamps  having  relatively  thick  filaments  can  be  worked 
at  lower  frequencies  than  those  of  the  common  sort,  so 
that  5O-volt  lamps,  particularly  of  large  candle-power, 
may  be  worked  at  30  cycles  or  thereabouts  rather  well,  and 
out  of  doors  even  down  to  25  cycles.  That  is,  at  a  pinch 
one  can  do  satisfactory  work  when  current  is  available  at 
25  cycles  or  so,  by  using  low  voltage  lamps  of  32,  50,  or 
100  cp,  which,  by  the  way,  are  capable  of  giving  admirable 
results  in  illumination  if  properly  disposed.  Of  course, 
such  practice  is  bad  in  point  of  efficient  distribution  of  cur- 
rent, but  on  occasion  it  may  be  useful. 

As  to  arc  lamps,  conditions  are  not  so  favorable.  The 
fluctuations  of  an  alternating  arc  lamp  are  easily  detected, 
even  at  60  cycles,  by  moving  a  pencil  or  the  finger  quickly 
when  strongly  illuminated.  The  effect  is  a  series  of 
images  along  the  path  of  motion,  corresponding  to  the 
successive  maxima  of  light  in  the  arc.  At  40  to  45  cycles 
the  flickering  becomes  evident  even  when  viewing  station- 
ary objects,  the  exact  point  where  trouble  begins  depend- 
ing upon  the  adjustment  of  the  lamp,  the  hardness  of  the 
carbons,  and  various  minor  factors.  Enclosing  the 
arc  mitigates  the  difficulty  somewhat,  but  does  not  re- 
move it. 

In  working  near  the  critical  frequency  the  best  results 
are  attained  by  using  an  enclosed  arc  lamp  taking  all  the 
current  the  inner  globe  will  stand,  with  as  short  an  arc  as 
will  work  steadily. 

When  polyphase  currents  are  available,  as  is  usually  the 
case  where  rather  low  frequencies  are  involved,  some  relief 
may  be  obtained  by  arranging  the  arcs  in  groups  consist- 
ing of  one  from  each  phase.  At  a  little  distance  from 
such  a  group  the  several  illuminations  blend  so  as  to  par- 
tially suppress  the  fluctuations  of  the  individual  arcs. 


LIGHT   AND    THE   EYE.  15 

This  device  makes  it  possible  to  obtain  fairly  satisfactory 
lighting  between  35  and  40  cycles.  At  these  frequencies, 
however,  the  arcs  should  not  be  used  except  when  a  very 
powerful  light  is  necessary,  or  when  the  slightly  yellowish 
tinge  of  incandescents  would  interfere  with  the  proper 
judgment  of  colors.  Powerful  incandescents  are  gener- 
ally better,  and  are  but  little  less  efficient,  particularly  when 
one  takes  into  account  proper  distribution  of  the  light. 
In  using  incandescents  in  large  masses,  particularly  on 
polyphase  circuits,  the  flickering  of  the  individual  lights 
is  lost  in  the  general  glow,  so  that  even  at  25  cycles  the 
light  may  be  steady  enough  for  general  purposes,  as  was 
the  case  with  the  decorative  lighting  at  the  Pan-American 
Exposition.  The  fluctuations  due  to  low  frequency  are 
usually  very  distressing  to  the  eye,  and  should  be  sedu- 
lously avoided.  Fortunately,  save  in  rare  instances,  the 
frequency  can  be  and  should  be  kept  well  above  the  dan- 
ger point. 

The  same  considerations  which  forbid  the  use  of  very 
intense  lights,  unshaded,  flickering  lights,  and  electric 
lights  at  too  low  frequency,  render  violent  contrasts  of 
brilliant  illumination  and  deep  shadows  highly  objection- 
able. It  should  be  remembered  that  in  daylight  the 
general  diffusion  of  illumination  is  so  thorough  that  such 
contrasts  are  very  much  softened,  even  in  full  sunlight, 
and  much  of  the  time  the  direct  light  is  modified  by  clouds. 
In  situations  where  the  sun  shines  strongly  down  through 
interstices  in  thick  foliage,  the  effect  is  decidedly  un- 
pleasant if  one  wishes  to  use  the  eyes  steadily,  and  if  in 
addition  the  wind  stirs  the  leaves  and  causes  flickering  the 
strain  upon  the  eyes  is  most  trying. 
Pin  artificial  lighting  one  should  carefully  avoid  the  con- 
ditions that  are  objectionable  in  nature,  which  can  easily 


1 6  THE  ART   OF   ILLUMINATION. 

be  done  by  a  little  foresight.  If  for  any  purpose  very 
strong  illumination  becomes  necessary  at  a  certain  point, 
the  method  of  furnishing  it  which  is  most  satisfactory 
from  a  hygienic  standpoint  is  to  superimpose  it  upon  a 
moderate  illumination  well  distributed.  If  a  brilliant 
light  is  needed  upon  one's  work,  start  with  a  fairly  well 
lighted  room  and  add  the  necessary  local  illumination,  in- 
stead of  concentrating  all  the  light  on  one  spot.  This 
procedure  avoids  dense  shadows  and  dark  corners,  and 
enables  the  eye  to  work  efficiently  in  a  much  stronger 
illumination  than  would  otherwise  be  practicable. 

It  should  not  be  understood  that  the  complete  abolition 
of  shadows  is  desirable.  On  the  contrary,  since  much  of 
our  perception  of  form  and  position  depends  upon  the 
existence  of  shadows,  the  entire  absence  of  them  is 
troublesome  and  annoying.  This  is  probably  due  to  two 
causes.  First,  the  absence  of  shadows  gives  an  appear- 
ance of  flatness,  out  of  which  the  eye  vainly  struggles  to 
select  the  wonted  degrees  of  relief.  In  a  shadowless  space 
we  have  to  depend  upon  binocular  vision  to  locate  points 
in  three  dimensions,  and  the  strain  upon  the  attention  is 
severe  and  quickly  felt. 

iSecond,  the  existence  of  a  shadowless  space  presupposes 
a  nearly  equal  illumination  from  all  directions.  If  it  be 
strong  enough  from  any  particular  direction  to  be  con- 
venient for  work  requiring  close  attention  of  mind  and 
eye,  then,  if  there  be  no  shadows,  equally  strong  light  will 
enter  the  eye  from  directions  altogether  unwonted.  This 
state  of  things  we  have  already  found  to  be  objectionable 
in  the  highest  degree. 

The  best  illustration  of  this  latter  condition  may  be 
found  in  nature  during  a  thin  fog  which  veils  the  sun  while 
diffusing  light  with  very  great  brilliancy.  Try  to  read  at 


LIGHT   AND    THE    EYE.  17 

such  a  time  out  of  doors,  and  although  there  is  no  direct 
light  on  the  page  to  dazzle  you,  and  there  is  in  reading  no   r 
trouble  from  the  sense  of  flatness,  yet  there  is  a  distinctly 
painful  glare  which  the  eyes  cannot  long  endure  without 
serious  strain. 

In  artificial  lighting  the  same  complete  diffusion  is  com- 
petent to  cause  the  same  results,  so  that  while  contrasts  of 
dense  shadows  and  brilliant  light  must  be  avoided,  it  is 
generally  equally  important  to  give  the  illumination  a 
certain  general  direction  to  relieve  the  appearance  of  flat- 
ness and  to  save  the  eye  from  crosslights.  S 

With  respect  to  the  best  direction  of  illumination,  only 
very  general  suggestions  can  be  given.  Brilliant  light,/ 
direct  or  reflected,  should  be  kept  out  of  the  eye  and  upon 
the  objects  to  be  illuminated.  In  each  individual  case  the 
nature  and  requirements  of  the  work  must  determine  the 
direction  of  lighting. 

The  old  rule  given  for  reading  and  writing,  that  the 
light  should  come  obliquely  over  the  left  shoulder,  well 
illustrates  ordinary  requirements.  By  receiving  the  light 
from  the  point  indicated  direct  light  is  kept  out  of  the  eyes, 
and  any  light  regularly  reflected  is  generally  out  of  the 
way.  The  eye  catches  then  only  diffused  light  from  the 
paper  before  it,  and  if  the  light  comes  from  the  left  (for  a 
right-handed  person)  the  shadow  of  hand  and  arm  does 
not  interfere  with  vision.  If  work  requiring  both  hands 
is  under  way  the  chances  are  that  the  best  illumination  will 
be  obtained  by  directing  it  downwards  and  slightly  from 
the  front,  in  which  case  care  must  be  exercised  to  avoid 
strong  direct  reflection  into  the  eyes.  The  best  simple  ^ 
rule  is,  avoid  glare  direct  or  reflected,  and  get  strong  dif- 
fused lio-ht  from  the  object  illuminated. 

This  brings  us  at  once  to  the  very  important  but  ill- 


i8  THE   ART   OF   ILLUMINATION. 

defined  question  of  the  strength  of  illumination  required 
for  various  kinds  of  work. 

(^Fortunately,  the  eye  works  well  over  a  wide  range  of 
brightness,  but  there  is  a  certain  minimum  illumination 
which  should  be  exceeded  if  one  is  to  work  easily  and 
without  undue  strain.  The  matter  is  much  complicated 
by  questions  of  texture  and  color,  which  will  be  taken  up 
presently,  so  that  only  general  average  results  can  be  con- 
sidered. For  reading  and  writing  experience  has  shown 
that  an  intensity  of  about  one  candle-foot  is  the  minimum  ^- 
suitable  amount  with  ordinary  type  and  ink,  such  as  is 
here  used,  for  instance.  With  large,  clear  type 

like  that  used  for  this  particular  line 

half  a  candle-foot  enables  one  to  read  rather  easily,  while 
with  ordinary  type  set  solid  or  in  type  of  the  smaller  sizes, 

such  type  as  is  employed  in  this  line  as  a  horrible  example, 

two  candle- feet  is  by  no  means  an  unnecessary  amount  of 
lighting.  Dense  black  ink  and  clear  white  paper  not 
highly  calendered,  such  as  some  of  the  early  printers  knew 
well  how  to  use,  make  vastly  easier  reading  than  the 
grayish-white  stuff  and  cheap  muddy-looking  ink  to  be 
found  in  the  average  newspaper. 

Illumination  of  less  than  half  a  candle-foot  usually  ren- 
ders reading  somewhat  difficult  and  slow,  the  more  diffi- 
cult and  slower  as  the  illumination  is  further  reduced. 
At  one-tenth  or  two-tenths  of  a  candle-foot  reading  is  by 
no  means  easy,  and  there  is  a  strong  tendency  to  bring  the 
book  near  the  eye,  thereby  straining  one's  power  of  ac- 
commodation, and  to  concentrate  the  attention  upon 
single  words,  a  tendency  which  increases  as  the  light  is 
still  further  lessened. 

In  fact,   when  the  illumination  falls  to  the  vicinity  of 


LIGHT   AND    THE   EYE. 


one-tenth  candle- foot  it  is  of  very  little  use  for  the  purpose 
of  reading  or  working.  *^J 

One  may  get  a  fair  idea  of  the  strength  of  illumination 
required  for  various  purposes  by  a  consideration  of  that 
actually  furnished  by  nature.  To  get  at  the  facts  in  the 
case,  we  must  make  a  little  digression  in  the  direction 'of 
photometry,  a  subject  which  will  be  more  fully  discussed 
later. 

To  get  an  approximate  measure  of  the  illumination  fur- 
nished by  daylight,  one  can  conveniently  use  what  is 


Fig.  4. — Principle  of  the  Photometer. 

known  as  a  daylight  photometer.  This  instrument  fur- 
nishes a  means  for  balancing  the  illumination  due  to  any 
source  against  that  due  to  a  standard  candle  at  a  known 
distance.  Like  most  common  forms  of  photometer  it 
consists  of  a  screen  illuminated  on  its  two  sides  by  the  two 
sources  of  light  respectively.  Equality  of  illumination  is 
determined  by  the  disappearance  of  a  grease  spot  upon  the 
screen.  A  spot  of  grease  on  white  paper  produces,  as  is 
well  known,  a  highly  transparent  spot,  which  looks  bright 
if  illuminated  from  behind,  and  dark  when  illuminated 
from  the  front. 

Thus,  if  one  sets  up  such  a  screen  C  between  and  equi- 


20  THE   ART   OF   ILLUMINATION. 

distant  from  a  candle  A  and  an  incandescent  lamp  B,  and 
then  looks  at  the  screen  obliquely  from  the  same  side  as 
B,  the  appearance  is  that  shown  in  Fig.  4.  Moving 
around  to  the  other  side  of  the  screen  one  gets  the  effect 
shown  in  Fig.  5.  By  moving  the  candle  A  nearer  or  the 
incandescent  B  farther  off,  a  point  will  be  found  where  the 
spot  becomes  nearly  invisible  on  account  of  the  equal 
illumination  on  the  two  sides.  This  "  Bunsen  photometer 
screen  "  requires  very  careful  working  to  get  highly  accu- 
rate results,  but  gives  closely  approximate  figures  readily. 


Fig.  5. — Principle  of  the  Photometer. 

The  daylight  photometer,  Fig.  6,  is  the  simplest  sort  of 
adaptation  of  this  principle.  It  consists  of  a  box,  say  five 
or  six  feet  long  and  fifteen  inches  square.  In  one  end  is 
a  hole  B  filled  with  the  photometer  screen  just  described, 
and  a  slot  to  receive  a  graduated  scale  A  carrying  a  socket 
for  a  standard  candle.  The  interior  of  the  box  is  painted 
dead  black,  so  as  to  avoid  increasing  the  illumination  at  B 
by  light  reflected  within  the  box. 

Setting  up  the  box  with  the  end  B  pointing  in  the  direc- 
tion of  the  illumination  to  be  estimated,  the  candle  is  slid 
back  and  forth  until  the  grease  spot  disappears,  when  the 


LIGHT    AND    THE    EYE.  21 

distance  from  the  candle  to  B  gives  the  required  illumina- 
tion, by  applying  the  law  of  inverse  squares,  which  holds 
sufficiently  well  for  approximate  purposes  if  the  box  is 
well  blackened. 

Of  course  the  results  of  such  measurements  vary  enor- 
mously with   different  conditions  of  daylight.     A   few 


Fig.  6. — Daylight  Photometer. 

* 

measurements  made  in  a  large,  low  room  with  windows  on 
two  sides,  culled  from  the  writer's  notebook,  give  the 
following  results,  the  day  being  bright,  but  not  sunny,  and 
the  time  early  in  the  afternoon : 

Facing  south  window 6  candle-feet 

Facing  east  window 2.2 

Facing  north  wall 0.7 

And  again,  10  feet  from  south  window,  on  a  misty 
April  day,  5  P.  M 0.5 

On  a  clear  day  the  diffused  illumination  near  a  window, 
while  the  sun  is  still  high,  will  generally  range  from  5  to 
10  candle- feet,  while  in  cases  where  there  are  exception- 
ally favorable  conditions  for  brilliant  illumination  it  may 
rise  to  twice  or  even  four  times  the  amount  just  stated. 

Now,  these  figures  for  the  lighting  effects  of  diffused 
daylight  give  a  good  clew,  if  nothing  more,  to  the  intensity 
of  illumination  required  for  various  purposes.  In  point 
of  fact,  reading  and  writing  require  less  light  than  almost 
any  other  processes  which  demand  close  ocular  attention. 


22  THE   ART   OF   ILLUMINATION. 

Everything  is  black  and  white,  there  is  no  delicate  shad- 
ing of  colors,  nor  any  degrees  of  relief  to>  be  perceived  in 
virtue  of  differences  of  light  and  shade.  Moreover,  the 
characters  are  sharply  defined  and  not  far  from  the  eye. 
It  is  therefore  safe  to  say  that  for  any  work  requiring 
steady  use  of  the  eyes  at  least  one  candle-foot  is  demanded. 
If  practicable,  this  minimum  should  be  doubled  for  really 
effective  lighting,  while  for  much  fine  detail  and  for  work 
on  colored  materials  not  less  than  five  candlesfeet  shOul'd 
be  provided.  Even  this  amount  may  advantageously  be 
doubled  for  the  finest  mechanical  work,  such  as  engraving, 
watch  repairing,  and  similar  delicate  operations.  In  fact, 
for  such  cases  the  more  light  the  better,  provided  the 
source  of  light  and  direct  undiffused  rjeflections  therefrom 
are  kept  out  of  the  eyes. 

These  estimates  have  taken  no  account  of  the  effect  of 
color,  which  sometimes  is  a  most  important  factor,  alike  in 
determining  the  amount  of  illumination  necessary  and  in 
prescribing  the  character  and  arrangement  of  the  sources 
of  light  to  be  employed. 


CHAPTER  II. 

PRINCIPLES    OF    COLOR. 

THE  relation  of  color  to  practical  illumination  is  some- 
what intricate,  for  it  involves  considerations  physical, 
physiological,  and  aesthetic,  but  it  is  well  worth  studying, 
for  while  in  some  departments  of  illumination,  such  as 
street  lighting,  it  is  of  little  consequence,  in  lighting  in- 
teriors it  plays  a  very  important  part.  In  lighting  a  shop 
where  colored  fabrics  are  displayed,  for  example,  it  is 
necessary  to  reproduce  as  nearly  as  may  be  the  color 
values  of  diffused  daylight,  even  at  considerable  trouble. 
Such  illumination,  however,  may  be  highly  undesirable  in 
lighting  a  ballroom,  where  the  softer  tones  of  a 
light  richer  in  yellow  and  orange  are  generally  far  prefer- 
able. 

In  certain  sorts  of  scenic  illumination  strongly  colored 
lights  must  be  employed,  but  always  with  due  understand- 
ing of  their  effect  on  neighboring  colored  objects.  Some- 
times, too,  the  natural  color  of  a  light  needs  to  be  slightly 
modified  by  the  presence  of  tinted  shades,  serving  to 
modify  both  the  intrinsic  brilliancy  and  the  color. 

The  fundamental  law  with  respect  to  color  is  as  follows : 
Every  opaque  object  assumes  a  hue  due  to  the  sum  of  the 
colors  which  it  reflects.  A  red  book,  for  instance,  looks 
red  because  from  white  light  it  selects  mainly  the  red 
for  reflection,  while  strongly  absorbing  the  green  and 
blue. 


24  THE   ART    OF   ILLUMINATION. 

White  light,  as  a  look  through  a  prism  plainly  shows,  is 
a  composite  of  many  colors,  fundamentally  red,  green,  and 
blue,  incidentally  of  an  almost  infinite  variety  of  transi- 
tion tints.  If  a  narrow  beam  of  sunlight  passes  through 
a  prism  it  is  drawn  out  into  a  many-colored  spectrum  in 
which  the  three  colors  mentioned  are  the  most  prominent. 
Closer  inspection  detects  a  rather  noticeable  orange  region 
passing  from  red  to  green  by  way  of  a  narrow  space  of 
pure  yellow,  which  is  never  very  conspicuous.  The  green 
likewise  shades  into  pure  blue  through  a  belt  of  greenish 
blue,  and  the  blue  in  turn  shades  off  into  a  deep  violet.  If 
the  slit  which  admits  the  sunlight  is  made  very  narrow, 
certain  black  lines  appear  crossing  the  spectrum — the 
Fraunhofer  lines  due  to  the  selective  absorption  of  vari- 
ous substances  in  the  solar  atmosphere,  These  lines  are 
for  the  purpose  in  hand  merely  convenient  landmarks  to 
which  various  colors  may  be  referred.  They  were  desig- 
nated by  Fraunhofer  by  the  letters  of  the  alphabet,  begin- 
ning at  the  red  end  of  the  spectrum. 

Fig.  7  shows  in  diagram  the  solar  spectrum  with  these 
lines  and  the  general  distribution  of  the  colors.  The  A 
line,  really  a  broad  dark  band  of  many  lines,  is  barely  visi- 
ble save  in  the  most  intense  light,  and  the  eye  can  detect 
little  or  nothing  beyond  it.  At  the  other  end  of  the  spec- 
trum the  H  lines  are  in  a  violet  merging  into  lavender,  are 
not  easy  to  see.  and  there  is  but  a  narrow  region  visible  be- 
yond them — pale  lavender,  as  generally  seen.  The  spec- 
trum in  Fig.  7  is  roughly  mapped  out  to  show  the  extent 
of  the  various  colors  as  distributed  in  the  ordinary  pris- 
matic spectrum. 

At  A,  Fig.  7,  is  shown  the  spectrum  of  the  light  reflected 
from  a  bright  red  book.  i.  e.,  the  color  spectrum  which  de- 
fines that  particular  red.  It  extends  from  a  deep  red  into 


PRINCIPLES   OF   COLOR. 


25 


clear  orange,  while  the  absorption  in  the  yellow  and  yel- 
lowish green  is  by  no  means  complete. 

At   B,    is    the   color    spectrum    from   a   green    book. 
Here  there  is  considerable  orange  and  yellow,  a  little  red 


~r 


r 


Fig.  7. — Solar  and  Reflected  Spectra. 

and  much  bright  green,  together  with  rather  weak  absorp- 
tion in  the  bluish  green. 

C  shows  a  similar  diagram  from  a  book  apparently  of  a 
clear,  full  blue.  The  spectrum  shows  pretty  complete  ab- 
sorption in  the  red  and  extending  well  into  the  orange. 
The  orange-yellow  and  yellowish-green  remain,  however, 
as  does  all  the  deep  blue,  while  there  is  a  perceptible  ab- 
sorption of  the  green  and  bluish-green. 

Now,  these  reflected  spectra  are  thoroughly  typical  of 
those  obtained  from  any  dyed  or  painted  surfaces.  The 
colors  obtained  from  pigments  are  never  the  simple  hues 
they  appear  to  be,  but  mixtures  more  or  less  complex, 
sometimes  of  colors  from  very  different  regions  of  the 
spectrum.  Most  of  the  commoner  pigments  produce  ab- 


26  THE   ART   OF   ILLUMINATION. 

sorption  over  rather  wide  regions  of  the  spectrum,  but 
some  of  the  delicate  tints  found  in  dyed  fabrics  show  sev- 
eral bands  of  absorption  in  widely  separated  portions  of 
the  spectrum.  These  are  the  colors  most  seriously  affected 
by  variations  in  the  color  of  the  illuminant  when  viewed  by 
artificial  light.  Fig.  8  is  a  case  in  point,  a  color  spectrum 
taken  from  a  fabric  which  in  daylight  was  a  delicate  corn- 
flower blue.  The  absorption  begins  in  the  crimson,  leav- 
ing much  of  the  red  intact,  is  partial  in  the  orange  and 
yellow,  stronger  in  the  green,  and  quite  complete  in  the 
bluish-green  region.  The  blue  well  up  to  the  violet  is 
freely  reflected,  and  then  the  violet  end  of  the  spectrum  is 


Fig.  8. — Spectrum  Reflected  From  Blue  Silk. 

considerably  absorbed.  Most  of  the  reflected  light  is  blue, 
but  if  the  illumination  is  conspicuously  lacking  in  blue 
rays,  as  is  the  case  with  candle  light  or  common  gaslight, 
the  blue  light  reflected  is  necessarily  weak,  while  the  red 
component  comes  out  at  its  full  strength,  and  the  visible 
color  of  the  fabric  is  distinctly  reddish. 

A  similar  condition  is  met  in  certain  blues  which  in  day- 
light reflect  a  large  proportion  of  blue  and  bluish  violet, 
but  in  which  some  green  rays  are  left,  just  as  was  the  clear 
red  in  Fig.  8.  By  gaslight  the  blue  becomes  relatively 
very  much  weakened,  and  the  apparent  color  is  unmistak- 
ably green.  Such  changes  in  hue  are  in  greater  or  less  de- 
gree very  common,  and  furnish  some  very  curious  effects. 
Sometimes  a  color  clear  by  daylight  appears  dull  and 


PRINCIPLES    OF   COLOR. 


27 


muddy  by  artificial  light,  and  in  general  the  quality  of  the 
illumination  requires  careful  attention  whenever  one  deals 
with  delicate  colors. 

The  absorption  found  in  the  pigments  used  in  painting 
is  seldom  so  erratic  as  that  shown  in  Fig.  8,  but  pictures 
often  show  very  imperfectly  under  ordinary  artificial 
illumination. 

It  is  no  easy  matter  to  get  a  clear  idea  of  the  color 
properties  of  various  illuminants.  O£  course,  one  can 
form  spectra  from  each  of  the  lights  to  be  compared,  and 
compare  the  relative  strengths  of  the  red,  green,  blue,  and 
other  rays  in  each,  but  this  gives  but  an  imperfect  idea  of 
the  relative  color  effects  produced,  for  the  results  them- 
selves are  rather  discordant,  and  the  relative  brightness 
thus  measured  does  not  correspond  accurately  with  the 
visual  effect.  Probably  a  better  plan  from  the  standpoint 
of  illumination  is  to  match  the  visible  color  of  a  given 
illuminant  accurately  by  mixtures  of  the  three  primary 
spectral  colors,  red,  blue-violet  and  green,  and  to  deter- 
mine the  exact  proportions  of  each  constituent  required  to 
give  a  match.  Even  this  evidently  does  not  tell  the  whole 
story,  but  it  gives  an  excellent  idea  of  the  color  differences 
found  in  various  lights.  Such  work  has  been  very  beauti- 
fully carried  out  by  Abney,  from  whose  results  the  follow- 
ing table  is  taken : 


SUNLIGHT 

SKY  LIGHT 

ARCLIGHT 

GASLIGHT 

Red  

IOO 

IOO 

IOO 

IOO 

Green  ... 

IQ-2 

2^6 

2O  -5 

05 

Violet 

228 

760 

2CQ 

27 

Incandescent  lamps  are  not  here  included,  but  give  enor- 
mously different  results  according  to  the  degree  of  incan- 


28  THE   ART   OF   ILLUMINATION. 

descence  to  which  they  are  carried.  If  burned  below 
candle-power  they  give  a  light  not  differing  widely  from 
gaslight;  while  if  pushed  far  above  candle-power  the  light 
is  far  richer  in  violet  rays,  and  becomes  pure  white. 
Unfortunately,  however,  the  lamp  does  not  reach  this 
point  save  at  a  temperature  that  very  quickly  ends  its  life. 

The  effects  of  the  selective  absorption  which  so  deceives 
the  eye  when  colored  objects  are  viewed  in  colored  lights 
are  shown  in  a  variety  of  ways  according  to  the  colors  in- 
volved, but  the  net  result  of  them  all  is 'to  show  the  neces- 
sity of  looking  out  for  the  color  of  artificial  lights.  Of 
course,  a  really  strong  color  may  produce  very  fantastic 
results.  For  example,  in  the  rays  of  an  ordinary  green 
lantern,  such  as  is  used  for  railway  signals,  greens  gener- 
ally appear  of  nearly  their  natural  hues;  but  greens,  yel- 
lows, browns,  and  grays  all  match  pretty  well,  although 
they  may  appear  darker  or  lighter  in  shade.  Pink  looks 
gray,  darkening  in  shade  as  it  gets  redder,  and  red  is 
nearly  black,  for  the  green  light  which  falls  upon  it  is  al- 
most totally  absorbed. 

Practical  illuminants  do  not  often  present  so  violent 
deceptions,  and  yet  gas  or  candle  light  is  certain  to  change 
the  apparent  hue  of  any  delicate  colors  containing  bluish 
green,  blue,  or  violet  rays.  An  old  Welsbach  mantle 
which  gives  a  light  of  a  strongly  greenish  cast  is  pretty 
certain  to  change  the  color  of  everything  not  green  upon 
which  it  falls.  Incandescent  electric  lights  affect  colors 
in  much  the  same  way  as  brilliant  gaslight,  while  arc 
lights  give  a  fair  approximation  to  daylight.  It  by  no 
means  follows,  however,  that  all  colors  should  be  matched 
by  arc  lights  in  preference  to  other  sources  of  illumina- 
tion. A  match  so  made  stands  daylight,  but  may  be  most 
faulty  when  viewed  by  gaslight.  * 


PRINCIPLES    OF   COLOR.  29 

If  matching  colors  has  to  be  done,  it  is  a  safe  rule  to 
match  them  by  the  kind  of  light  by  which  they  are  in- 
tended to  be  viewed.  Moreover,  different  shades  of  the 
same  color  are  differently  affected  in  artificial  light.  As  a  v 
rule,  deep,  full  colors  are  far  less  affected  than  light  tones 
of  the  same  general  hue.  Clear  yellows,  reds,  and  blues 
not  verging  on  green  are  usually  little  altered,  but  pale 
pinks,  violets,  and  "  robin's-egg "  blues  quite  generally 
suffer.  Very  often  when  a  color  is  not  positively  altered 
it  is  made  to  appear  gray  and  muddy. 

For  while  in  a  green  light  greens  look  particularly  bril- 
liant, red  may  be  practically  extinguished,  absorbing  all 
the  rays  which  come  to  it,  so  that  a  deep  red  will  be  nearly 
black,  and  a  very  light  red  merely  a  dirty  white,  tinged 
with  green  if  anything. 

Quite  apart  from  any  effect  of  colored  illumination, 
colors  seem  to  change  in  very  dim  light.  This  is  a  purely 
physiological  matter,  the  eye  itself  differing  in  its  sensi- 
bility to  different  colored  lights.  In  very  faint  illumina- 
tion no  color  of  any  kind  is  perceptible — everything  ap- 
pears of  uncertain  shades  of  gray.  As  the  light  fades 
from  its  normal  intensity,  as  in  twilight,  red  disappears 
first,  then  violet  and  deep  blue  follow,  settling  like  the  red 
into  murky  blackness;  then  the  bluish  green  and  green 
shade  off  into  rapidly  darkening  gray,  and  finally  the  yel- 
low and  yellowish  orange  lose  their  identity  and  merge 
into  the  night.  At  the  same  time  the  hues  even  of  simple 
colors  change,  scarlet  fading  into  orange,  orange  into  yel- 
low, and  green  into  bluish  green. 

Obviously,  complicated  composite  colors  must  vary 
widely  under  such  circumstances,  for  as  the  light  grows 
dimmer  their  various  components  do  not  fade  in  equal 
measure.  Pinks,  for  instance,  generally  turn  bluish  gray 


3o  THE   ART   OF   ILLUMINATION. 

at  a  certain  stage  of  illumination,  owing  to  the  extinction 
of  the  red  rays.  In  fact,  in  a  dim  light  the  normal  eye 
is  color  blind  as  regards  red,  and  one  can  get  a  rather 
good  idea  of  the  sensations  of  the  color  blind  by  study- 
ing a  set  of  tinted  wools  or  slips  of  paper  in  the  late 
twilight. 

The  similarity  of  the  conditions  is  strikingly  illustrated 
in  Fig.  9,  which  shows  in  No.  i  the  distribution  of  lumi- 

i    2       3 


.100 
90 
80 
70 
60 
50 
40 
30 
20 
10 
0 


\/ 


\\ 


\\ 


\ 


ABCD  E6          F  T  G  H^ 

Fig.  9.— Effect  of  Faint  Light  on  Color. 

nosity  in  the  spectrum  of  bright  white  light  to  the  normal 
eye,  and  in  No.  2  the  luminosity  of  the  same  as  seen  by  a 
red-blind  eye.  No.  3  shows  the  luminosity  of  the  spec- 
trum when  reduced  to  a  very  small  intensity  and  seen  by 
the  normal  eye.  The  data  are  from  Abney's  experiments, 
and  the  intensity  of  No.  3  was  such  that  the  yellow  com- 
ponent of  the  light  corresponding  to  D  of  the  spectrum 
was  0.006  candle-foot.  The  ordinates  of  No.  2  and  No. 
3  have  been  multiplied  by  such  numbers  as  would  bring 
their  respective  maxima  to  equal  the  maximum  of  No.  i, 


PRINCIPLES    OF    COLOR.  31 

as  the  purpose  is  to  show  their  relative  shapes  only.  The 
"  red-blind  "  curve  No.  2  shows  very  faint  luminosity  in 
the  scarlet  and  orange  and  absence  of  sensation  in  the 
crimson,  while  the  maximum  luminosity  is  in  the  greenish 
yellow.  It  is  easy  to  see  that  the  sensation  of  red  is  prac- 
tically obliterated. 

But  in  No.  3  every  trace  of  red  is  gone,  and  the  maxi- 
mum brilliancy  has  moved  up  into  the  clear  green  of  the 
spectrum  at  the  line  E.  With  a  still  further  reduction  of 
intensity,  the  spectrum  would  fade  into  gray  as  just  noted, 
while  a  slight  increase  of  light  would  cause  No.  3  closely 
to  approximate  No.  2. 

Starting  with  the  normal  curve  of  luminosity  No.  i,  the 
peak  of  the  curve  being  one  candle-power,  the  light  at  B 
would  disappear  if  the  illumination  were  reduced  to  .01 
of  its  initial  value,  that  at  C  at  about  .001 1,  at  D  .00005,  at 
E  .0000065,  at  F  .000015,  arjd  at  G  .0003. 

Now  the  practical  application  of  these  facts  is  manifold. 
Not  only  do  they  explain  the  odd  color  effects  at  twilight 
and  dawn,  but  it  is  worth  noting  that  the  cold  greenish 
hue  of  moonlight  on  a  clear  night  means  simply  the  abr 
sence  of  the  red  and  orange  from  one's  perception  of  a 
very  faint  light,  for  dim  moonlight  is  ordinarily  not  much 
brighter  than  the  light  of  curve  No.  3.  For  the  same  rea- 
son a  red  light  fades  out  of  sight  rather  quickly,  so  that  a 
signal  of  that  color  is  not  visible  at  a  distance  at  which 
one  of  another  color  and  equal  brightness  would  be  easily 
seen. 

Not  only  is  the  eye  itself  rather  insensitive  to  red,  but 
the  luminosity  of  the  red  part  of  the  spectrum  of  any  light 
is  rather  weak,  so  that  when  the  other  rays  are  cut  off  by 
colored  glass,  the  effective  light  is  greatly  reduced. 
About  87  per  cent,  of  the  effective  luminosity  of  white 


32  THE  ART   OF  ILLUMINATION. 

light  lies  between  the  lines  C  (scarlet)  and  E  (deep 
green),  the  relative  luminosities  at  various  points  being 
about  as  follows : 


LINE.  LUMINOSITY. 

B  3 

C  20 

D  98.5 

E  50 

b  35 

F  7 

G  0.6 


The  luminosities  of  light  transmitted  through  ordi- 
nary colored  glasses  of  various  colors  is  about  as  fol- 
lows, following  Abney's  experiments,  clear  glass  being 


COLOR  OF  GLASS.  LIGHT  TRANSMITTED. 

Ruby 13-1 

Canary 82.0 

Bottle  green 10.6 

Bright  green  (signal  green  No.  2) 19.4 

Bluish  green  (signal  green  No.  i) 6.9 

Cobalt  blue 3-75 

These  figures  emphasize  the  need  of  a  very  powerful 
source  if  it  is  necessary  to  get  a  really  bright-colored 
flight.  It  is  worth  noting  that  red  is  a  particularly 
bad  color  for  danger  signals  on  account  of  its  low  lumi- 
nous effect,  and  were  it  not  for  the  danger  of  changing  a 
universal  custom,  red  should  be  the  "  clear  "  signal  and 
green  the  danger  signal,  the  latter  color  giving  a  much 
brighter  light,  and  thus  being  on  the  average  more  easily 
visible. 

It  is  easy  to  see  that  any  artificial  illuminant  is  at  a  con- 
siderable disadvantage  if  at  all  strongly  colored,  for  not 
only  does  a  preponderance  of  red  or  green  rays  injure  color 
perception,  but  the  luminosity  of  such  rays  is  rather  low, 


PRINCIPLES    OF    COLOR.  33 

and  they  do  not  compensate  for  their  presence  by  giving 
greatly  increased  illumination. 

Owing  to  this  fact  the  effective  illumination  derived 
from  various  sources  of  light  is  pretty  nearly  proportional 
to  the  intensity  of  the  yellow  component  of  each.  Crova 
has  based  on  this  rule  an  ingenious  approximate  method 
of  comparing  the  total  intensity  of  colbred  lights  by  com- 
paring the  intensities  of  their  yellow  rays,  either  from 
their  respective  spectra  or  by  sifting  out  all  but  the  t 
yellow  and  closely  adjacent  rays  by  means  of  a  colored 
screen. 

Certainly  for  practical  purposes  the  rays  at  the  ends  of 
the  spectrum  are  not  very  useful.  So  far  as  the  ordinary 
work  of  illumination  goes,  white  or  yellowish  white 
light  should  be  used,  and  the  only  practical  function  oL 
strongly  colored  lights  is  for  signaling  and  scenic  illu- 
mination. 

The  general  effect  of  strongly  colored  lights  is  to  ac- 
centuate objects  colored  like  the  light  and  to  change  or  dim 
all  others.  Lights  merely  tinted  produce  a  similar  effect 
in  a  less  degree.  Bluish  and  greenish  tinges  in  the  light 
give  a  cold,  hard  hue  to  most  objects,  and  produce  on  the 
face  an  unnatural  pallor;  in  fact,  on  the  stage  they  are 
used  to  give  in  effect  the  pallor  of  approaching  dissolu- 
tion. Naturally  enough  such  light  is  unfitted  for  interior 
illumination,  as,  aside  from  its  effect  on  persons,  it  makes 
a  room  look  bare,  chill,  and  unfurnished.  In  a  less  degree  , 
a  similar  effect  is  produced  by  moonlight,  which,  from  a  ^ 
clear  sky,  is  distinctly  cold,  the  white  light  growing  faintly 
greenish  blue  as  its  diminishing  intensity  causes  the  red  to 
disappear. 

On  the  other  hand,  a  yellow-orange  tinge  in  the  light 
seems  to  soften  and  brighten  an  interior,  giving  an  effect 


34  THE   ART   OF   ILLUMINATION. 

generally  warm  and  cheery.  This  result  is  extremely 
well  seen  in  stage  fire-light  effects.  Strongly  red  light  is, 
however,  harsh  and  trying  and  particularly  difficult 
to  see  well  by,  so  that  it  should  generally  be  carefully 
avoided. 

While  it  is  not  easy  to  predict  accurately  the  effect  of 
tinted  lights  upon  various  delicate  shades  without  a  careful 
study  of  the  light  rays  forming  each,  the  average  effects 
relating  to  the  simpler  colors  are  summarized  in  the  fol- 
lowing table.  It  is  compiled  from  the  experiments  of  the 
late  M.  Chevreul,  for  many  years  director  of  the  dyeworks 
of  the  Gobelins  tapestries.  The  colored  lights  were  from 
sunlight  sifted  through  colored  glass,  and  the  effects  were 
upon  fabrics  dyed  in  plain,  simple  colors. 

The  facts  set  forth  in  this  table  show  well  what  should 
be  avoided  in  colored  illumination.  As  regards  various 
shades  of  the  same  colors  it  must  be  remembered  that  light 
shades  are  merely  the  full  deep  ones  diluted  with  white, 
which  is  itself  affected  by  the  color  of  the  incident  light. 
In  a  general  way,  therefore,  one  can  use  this  table  over  a 
wider  range  than  that  written  down. 

For  instance,  a  very  light  red  in  blue  light  would  look 
blue  with  a  mere  trace  of  violet,  while  in  yellow  light  it 
would  be  bright  yellow  with  a  very  slight  orange  cast. 
Generally  a  very  light  color  viewed  by  colored  light  will 
be  between  the  effect  produced  on  the  full  color,  and  that 
produced  by  the  light  on  a  white  surface.  Similarly  a 
light  only  tinged  with  color  will  only  slightly  modify  the 
tone  of  a  colored  object  in  the  direction  indicated  for  the 
full-colored  light  in  the  table. 

But  delicate  shades  from  modern  dyestuffs,  which  often 
absorb  the  light  in  very  erratic  ways,  as  in  Fig.  8,  are  a 
different  matter,  and  do  not  obey  any  simple  laws.  On 


PRINCIPLES    OF    COLOR. 


35 


ORIGINAL 
COLOR  OF 
FABRIC 

Black 

COLOR  OF  LIGHT  FALLING   UPON  FABRICS 

RED 

ORANGE 

YELLOW 

GREEN 

BLUE 

VIOLET 

Purplish 
Black 

Deep 
Maroon 

Yellow 
Olive 

Greenish 
Brown 

Blue- 
Black 

Faint  Vio- 
let Black 

White 

Red 

Orange 

Light 
Yellow 

Green 

Blue 

Violet 

Red 
Orange 

Intense 
Red 

Orange 
Red 

Scarlet 

Intense 
Orange 

Orange 

Yellow- 
Orange 

Brown 

Faint  Yel- 
low slight- 
ly Green- 
ish 

Violet 
Brown 

« 

Red-Violet 
Purple 

Light  Red 

Yellow 

Orange 

Yellow 
Orange 

Orange- 
Yellow 

Yellowish 
Green 

Green 

Brown 
tinged 
with 
faint  Red 

Light 
Green 

Reddish- 
Gray 

Yellow 
Green 

Greenish 
Yellow 

Intenser 
Green 

Blue- 
Green 

Light 
Purple 

Deep 

Green 

Reddish 
Black 

Rusty 
Green 

Yellowish 
Green 

Intenser 
Green 

Greenish 
Blue 

Light 
Blue 

Violet 

Orange 
Gray 

Yellowish 
Green 

Green 

Blue 

Vivid  Blue 

Deep 

Blue 

Indigo 
Blue 

Gray 
slightly  on 
Orange 

Orange- 
Maroon 

Green- 
Slate 

Orange- 
Yellow 
(very  dull) 

Blue 
Green 

Dull 
Green 

Intenser 
Blue 

Dark 
Blue- 
Indigo 

Bright 
Blue- 
Violet 

Deep 
Blue- 
Violet 

Violet 

Purple 

Red- 
Maroon 

Yellow- 
Maroon 

Bluish 
Green- 
Brown 

Deep 

Bluish 
Violet 

Deep 
Violet 

the  other  hand,  pure  colors,  in  the  sense  in  which  the 
scarlet  around  the  C  line  of  the  spectrum  is  pure,  act  in  a 
fashion  rather  different  from  that  shown  in  the  table, 
which  pertains  to  standard  dyestuffs  which  never  are  any- 
where near  being  pure  colors.  However,  as  artificial 
illumination  has  to  do  only  with  commercial  pigments  and 
dyes,  the  table  serves  as  a  useful  guide  in  judging  the 
effects  produced  on  interior  furnishings  by  change  in  the 
color  of  the  light. 

Of  common  illuminants,  none  have  any  very  decided 


36  THE   ART   OF   ILLUMINATION. 

color,  yet  most  are  somewhat  noticeably  tinged.     One 
can  tabulate  them  roughly  as  follows : 

ILLUMINANT.  COLOR. 

Sun  (high  in  sky).  White. 

Sun  (near  horizon).  Orange  red. 

Sky  light.  Bluish  white. 

Electric  arc  (short).  White. 

Electric  arc  (long).  Bluish  white  to  violet. 

Nernst  lamp.  White. 

Incandescent  (normal).  Yellow-white. 

Incandescent  (below  voltage).  Orange  to  orange-red. 

Acetylene  flame.  Nearly  white. 

Welsbach  light.  Greenish  white. 

Gaslight  (Siemens  burner).  Nearly  white,  faint  yellow  tinge. 

Gaslight,  ordinary.  Yellowish  white  to  pale  orange. 

Kerosene  lamp.  Yellowish  white  to  pale  orange. 

Candle.  Orange  yellow. 

Outside  the  earth's  atmosphere  the  sun  would  look  dis- 
tinctly blue,  while  its  light,  after  thorough  absorption  in 
the  earth's  atmosphere,  gets  the  blue  pretty  completely 
sifted  out,  so  that  the  light  from  the  eclipsed  moon,  once 
refracted  by  the  earth's  atmosphere  and  then  reflected 
through  it  again,  is  in  color  a  deep  coppery  red. 

Arc  lights  vary  much  in  color,  from  clear  white 
in  short  arcs  with  comparatively  heavy  current  to  bluish 
white  or  whitish  violet  in  long  arcs  carrying  rather  small 
current.  The  modern  enclosed  arcs  tend  in  the  latter 
direction,  and  give  their  truest  color  effects  with  yellowish 
white  inner  globes  or.  shades.  Incandescents,  as  gener- 
ally worked,  verge  upon  the  orange.  Of  the  luminous 
flames  in  use,  only  acetylene  comes  anywhere  near  being 
white,  although  the  powerful  regenerative  burners  are  a 
close  second.  Incandescent  gas  lamps,  at  first  showing 
nearly  white  with  a  very  slight  greenish  cast,  acquire  a 
greenish  or  yellowish  green  tinge  after  burning  for  some 
time. 

It  is  evident  then  that  a  study  of  the  color  effects  pro- 


PRINCIPLES    OF   COLOR.  37 

duced  by  colored  illuminants  is  by  no  means  irrelevant,  for 
distinct  tinges  of  color  are  the  rule  rather  than  the  ex- 
ception. 

But  this  is  not  at  all  the  whole  story,  for  the  general 
color  of  the  illumination  in  a  given  space  depends  not  only 
on  the  hue  of  the  illuminant,  but  upon  the  color  of  the 
surroundings.  Colored  shades,  of  course,  are  in  common 
use;  sometimes  with  a  definite  purpose,  more  often  from  a 
mistaken  notion  of  prettiness.  Used  intelligently,  as  we 
shall  presently  see,  they  may  prove  very  valuable  adjuncts 
in  interior  illumination. 

But  far  more  important  than  shading  is  the  modification 
in  the  color  of  the  light  which  comes  from  selective  reflec- 
tion at  surfaces  upon  which  the  light  falls.  In  every  en- 
closed space  light  is  reflected  in  one  way  or  another  from 
all  the  bounding  surfaces,  and  at  each  reflection  not  only 
is  the  amount  of  light  profoundly  modified,  but  its  color 
may  undergo  most  striking  changes.  It  is  this  phenome- 
non that  gives  its  greatest  interest  to  the  study  of  color  in 
illumination.  Its  importance  is  not  always  readily  recog- 
nized, for  few  persons  pay  really  close  attention  to  the 
matter  of  colors,  but  now  and  then  it  obtrudes  itself  in  a 
way  that  forces  attention. 

Take  for  example  a  display  window  lined  with  red  cloth 
and  brightly  illuminated.  Passing  along  the  sidewalk 
one's  attention  is  immediately  drawn  to  a  red  glow  upon 
the  street,  while  the  lights  themselves  may  be  ordinary  gas 
jets.  To  get  at  the  significance  of  this  matter,  we  must 
take  up  the  effect  of  reflection  and  diffusion  in  modifying 
the  amount  and  quality  of  light. 


CHAPTER  III. 

REFLECTION    AND    DIFFUSION. 

To  begin  with,  reflection  is  of  two  kinds — in  their 
essence  the  same,  yet  exhibiting  very  different  sets  of 
properties.  The  first,  or  regular  reflection,  may  be  best 
exemplified  by  the  reflection  which  a  beam  of  light  under- 
goes at  the  surface  of  a  mirror.  The  beam  strikes  the 
surface  and  is  reflected  therefrom  as  sharp  and  as  distinct 
as  it  was  before  its  incidence,  and  in  a  perfectly  definite 
direction. 

The  character  of  this  regular  reflection  is  very  clearly 
shown  in  Fig.  10.  Here  B  is  the  reflecting  surface — a 
plane,  polished  bit  of  metal,  for  instance.  AB  is  the  inci- 
dent ray  and  BC  the  reflected  ray.  In  such  reflection  two 
principal  'facts  characterize  the  nature  of  the  phenomenon. 
In  the  first  place,  if  a  perpendicular  to  the  surface  of  the 
mirror — as  BH — -is  erected  at  the  point  of  incidence,  the 
angle  ABJ}  is  always  precisely  equal  to  the  angle  DBC.f 
In  other  words,  the  angle  of  incidence  is  equal  to  the  angle 
of  reflection,  which  is  the  first  law  of  regular  reflection. 
Moreover,  the  incident  ray  AB,  the  normal  to  the  surface* 
at  the  point  of  incidence  BD,  and  the  reflected  ray  BC  are 
aH  in  the  same  plane. 

In  this  ordinary  form  of  reflection,  such  as  is  familiar 
in  mirrors,  the  direction  of  the  reflected  ray  is  entirely 
determinate,  and,  in  general,  although  the  reflected  ray 
has  lost  in  intensity,  it  is  not  greatly  changed  in  color.  A 
polished  copper  surface,  to  be  sure,  shows  a  reddish  reflec- 

38 


REFLECTION   AND    DIFFUSION. 


39 


tion,  and  polished  gold  a  distinctly  yellowish  reflection. 
Only  in  certain  dye  stuffs  which  exhibit  a  brilliant  metallic 
reflection  is  the  color  strongly  marked.  In  other  words, 
a  single  reflection  from  a  good,  clean,  reflecting  surface 
does  not  very  greatly  ctiange  either  the  intensity  or  the 
color  of  the  reflected  beam.  The  angle  of  incidence 


Fig.  10. — Regular  Reflection. 

affects  the  brilliancy  of  the  reflection  somewhat,  but  the 
color  only  imperceptibly.  In  the  art  of  practical  illumina- 
tion regular  reflection  comes  into  play  only  in  a  rather 
helpful  way,  and  kindly  refrains  from  complicating  the 
situation  with  respect  to  color  or  intensity. 

The  second  sort  of  reflection  is  what  is  technically 
known  as  diffuse  reflection.  This  term  does  not  mean 
that  the  phenomenon  itself  is  of  a  totally  different  kind 
from  regular  reflection,  but  nevertheless,  its  results  are 
totally  different.  No  surface  is  altogether  smooth. 
Even  with  the  best  polished  metallic  mirrors,  while  the  re- 
flected image  is  perfectly  distinct  at  ordinary  angles  of 
reflection,  it  is  apt  to  become  slightly  hazy  at  grazing  inci- 


40  THE   ART   OF  ILLUMINATION. 

dence — that  is,  when  the  incident  and  reflected  beams  are 
nearly  parallel  to  the  surface.  This  simply  means  that 
under  such  conditions  the  infinitesimal  roughness  of  the 
reflecting  surface  begin  to  be  in  evidence. 

To  get  an  idea  of  the  nature  of  diffuse  reflection,  ex- 
amine Fig.  ii.  In  this  case  a  section  of  the  reflecting 
surface  is  rough,  showing  grooves  and  points  of  every 


Fig.  ii. — Diffuse  Reflecti($n. 

description — in  fact,  nearly  everything  except  a  plane 
surface.  Consider  now  the  effect  of  a  series  of  parallel 
incident  beams — numbered  in  the  figure  from  i  to  10 — 
falling  upon  the  surface.  Each  one  of  them  is  reflected 
from  its  own  point  of  incidence  in  a  perfectly  regular  man- 
ner; yet  the  reflected  rays,  on  account  of  the  irregularity 
of  the  surface,  lie  in  all  sorts  of  directions,  and  moreover, 
in  all  sorts  of  planes,  according  to  the  particular  way  in 
which  the  surface  at  the  point  of  incidence  is  distorted. 
Diffuse  reflection,  therefore,  scatters  the  incident  beam  in 
all  directions,  for  the  roughnesses  of  an  unpolished  surface 
are  generally  totally  devoid  of  any  regularity.  The  point 


REFLECTION   AND    DIFFUSION.  41 

of  incidence  upon  which  a  beam  falls,  therefore,  radiates 
light  in  a  diverging  cone  and  behaves  as  if  it  were  really 
luminous. 

Some  consideration  of  the  nature  of  this  diffuse  reflec- 
tion will  bring  to  light  a  fact  which  in  itself  seems  rather 
surprising:  namely,  that  the  total  intensities  of  the  two 
kinds  of  reflection  are  not  so  different  from  each  other  as 
might  appear  probable  at  first  thought — provided  the 
roughness  of  the  unpolished  surface  is  not  on  too  small  a 
scale;  for  each  of  the  incident  rays  in  Fig.  n  is  reflected 
from  the  surface  just  as  in  the  case  of  Fig.  10,  in  a  per- 
fectly clean,  definite  way,  and  there  is  no  intrinsic  reason 
why  the  intensity  of  this  elementary  ray  should  be  any 
more  diminished  than  in  the  case  of  regular  reflection. 

A  little  inspection  of  Fig.  n,  however,  shows  that  rays 
Nos.  5  and  10  are  twice  reflected  before  they  get  fairly 
clear  of  the  surface,  and  if  one  went  on  drawing  still  more 
incident  rays  and  following  out  the  figure  on  a  still  finer 
scale,  a  good  many  other  rays  would  be  found  to  be  re- 
flected two  or  more  times  before  finally  escaping  from  the 
surface.  Such  multiple  reflection,  of  course,  diminishes 
the  intensity  of  the  light  just  as  in  the  multiple  reflection 
from  mirrors,  for  there  is  always  a  little  absorption, 
selective  or  otherwise,  at  any  surface  however  apparently 
opaque.  Thus,  while  the  difference  in  the  final  intensi- 
ties of  light  regularly  and  diffusely  reflected  is  not  so  great 
as  might  be  imagined,  it  still  does  exist,  and  for  a  perfectly 
logical  reason. 

To  go  into  the  matter  a  little  further — suppose  the 
rough  surface  of  Fig.  n  to  be  not  heterogeneous,  but 
made  up  of  a  series  of  grooves  having  Cross- sect  ions  like 
saw  teeth.  On  examining  the  reflection  from  such  a 
surface  we  should  find  a  rather  remarkable  state  of  affairs, 


42  THE   ART    OF   ILLUMINATION. 

for  the  course  of  reflection  would  then  vary  very  greatly 
with  the  relation  between  the  direction  of  the  incident 
light  and  the  surfaces  of  the  grooves  in  the  reflecting 
surface. 

Light  coming  in  one  direction,  i.  e.,  so  as  to  strike  the 
inclined  surfaces  of  the  grooves,  would  get  clear  of  the 
surface  at  the  first  reflection,  and  the  intensity  of  the  re- 
flected beam  would  have  a  very  marked  maximum  in  one 
particular  direction.  A  beam  falling  on  the  reflecting 
surface  in  the  other  direction,  however — that  is,  on  the 
perpendicular  sides  of  the  saw-tooth  grooves,  would  suffer 
several  reflections  before  escaping  from  the  grooves,  and 
hence  would  lose  in  intensity,  might  be  changed  in  color, 
and  might  be  considerably  diffused.  This  sort  of  phe- 
nomenon one  may  call  asymmetric  reflection.  As  we 
shall  presently  see,  it  plays  a  somewhat  important  part  in 
some  very  familiar  phenomena. 

Reflection  from  ordinary  smooth  but  not  polished 
surfaces  partakes  both  of  the  nature  of  regular  and  diffuse 
reflection,  and  is,  in  fact,  a  mixture  of  the  two  phenomena, 
there  being  a  general  predominant  direction  of  reflection 
plus  a  certain  amount  of  diffuse  reflection.  This  sort  of 
thing  is  most  commonly  met  with  in  practical  illumina- 
tion. The  light  from  artificial  illuminants  usually  falls 
on  painted  walls,  on  tinted  papers  with  surfaces  more  or 
less  regular,  on  fabrics  and  on  various  rough  or  smooth 
objects  in  the  vicinity.  If  these  surrounding  surfaces  are 
colored — as  in  the  case  discussed  a  little  while  ago — some 
curious  results  may  be  produced.  Of  course,  light  re- 
flected from  a  colored  surface  is  colored,  as  we  have  seen 
already,  but  the  manner  in  which  it  is  colored  is  by  no 
means  obvious. 
1  When  white  light  falls  upon  a  colored  surface,  the  re- 


REFLECTION   AND    DIFFUSION. 


43 


flection  is  generally  highly  selective  as  regards  color. 
Fig.  12,  from  Abney's  data,  shows  clearly  enough  the  sort 
of  thing  which  occurs.  It  exhibits  the  intensity  of  the  re- 
flected light  in  each  part  of  the  spectrum  when  the  reflect- 
ing surface  is  colored.  The  surfaces  in  this  case  were 
smooth  layers  of  pigment.  Curve  No.  i  is  the  light  re- 


/\ 

' 

N, 

i 

\, 

/ 

/ 

\ 

^ 

•*x 

\ 

ix 

H 

»" 

a,    fiO 

V 

/ 

/ 

X 

\ 

/ 

\ 

/ 

^ 

I 

M    *n 

/ 

/H 

\ 

/ 

> 

\ 

\ 

*g    M' 

s>  10 

i 

/ 

\ 

^ 

/ 

'/ 

\ 

(3 

/ 

/ 

s 

"Si 

Sj 

/ 

/ 

*•" 

v^ 

^ 

^ 

on  . 

*•- 

i"**1 

i 

"* 

•>v 

^ 

^, 

/ 

•~- 

^ 

•^ 

10  • 

/ 

/ 

3 

,—  — 

t 

. 

3 

J 

r~    i 

) 

G 

Fig.  12. — Selective  Reflection. 

fleeted  from  a  surface  painted  cadmium-yellow;  No.  2, 
Antwerp  blue;  No.  3,  emerald  green.     Each  curve  shows 
a  principal  reflection  of  the  color  of  the  pigment,  reaching 
a  rather  high  maximum  value,  but  falling  off  rapidly  in 
parts  of  the  spectrum  other  than  that  to  which  the  pre- 
dominant pigment  color  belongs.     As  has  been  already 
shown,  pigment  colors  are  nearly  always  impure,  and  this 
fact  is  strikingly  exhibited  in  the  shape  of  the  curves.     It 
is  clear  enough  what  will  be  the  color  of  the  main  body  of 
Alight  reflected  from  any  one  of  these  surfaces. 
Vy  The  visible  color  of  the  light  is,  however,  strongly  in- 
/  fluenced  by  the  character  of  the  surface.     A  shiny  enamel 


\ 


44  THE   ART   OF   ILLUMINATION. 

paint,  for  example,  will  reflect  a  good  deal  of  light  which 
is  not  strongly  influenced  by  the  pigment,  but  is  reflected 
from  the  surface  of  the  medium  without  much  selective 
action;  consequently,  there  will  be  in  the  reflected  light 
both  light  which  has  taken  the  color  of  the  pigment  and 
light  unchanged  in  color.  In  other  words,  when  viewed  by 
reflected  light,  the  pigment  color  is  mixed  with  white,  and 
when  we  have  a  perfectly  simple  pigment  color — such  as 
is  not  found  in  practice — this  would  lead  merely  to  light- 
ening the  tint.  It  may,  however,  have  results  much  more 
far-reaching — for  an  admixture  of  white  light  in  sufficient 
quantity  would  shut  out  the  distinct  perception  of  any 
color,  diluting  it  until  it  becomes  invisible. 

The  effects  of  this  dilution  are  most  marked  in  the  ends 
of  the  spectrum — the  colors  at  the  middle  being  least 
affected  by  the  admixture  of  white  light;  hence,  the  fact 
that  such  a  surface  as  we  have  been  considering,  reflecting 
a  mixture  of  white  and  colored  light,  may  produce  a 
change  not  only  in  tint,  but  in  the  hue  of  the  color,  if  the 
color,  as  usual,  is  composite.  For  example,  a  purple  in 
enamel  paint  might — according  to  its  composition — look 
pinkish  or  light  blue  if  the  surface  reflection  of  white  light 
were  particularly  strong.  If  the  pigmented  surface  is  not 
shiny  and  capable  of  considerable  reflection  of  uncolored 
light,  another  phenomenon  may  appear. 

Fig.  13  shows  curve  No.  3  of  Fig.  12,  emerald  green 
pigment  and  below  it  a  similar  curve,  resulting  from  a 
second  reflection  of  the  light  selectively  reflected  from  a 
pigment  of  that  color.  Assuming  what  is  nearly  in  ac- 
cordance with  the  fact — that  the  second  reflection  follows 
closely  the  properties  of  the  first — the  result  is  obviously 
to  intensify  the  green  of  the  reflected  light.  The  clear 
green  portion  of  the  light  reflected  from  this  particular 


REFLECTION   AND    DIFFUSION. 


45 


pigment  is  practically  embraced  between  the  dotted  lines 
P  and  Q  of  Fig.  13.  After  one  reflection  the  area  under 
the  curve  embraced  by  these  two  lines  is  about  42  per  cent, 
of  the  whole.  After  two  reflections  it  has  risen  to  55  per 
cent.,  and  each  successive  reflection — while  greatly  reduc- 


Fig.  13. — Effect  of  Multiple  Reflection. 


-will 


ing  the  intensity  of  the  reflected  light  as  a  whol< 
leave  it  greener  and  greener. 

Consequently  in  diffuse  reflection  those  rays  which  are 
reflected  several  times  before  escaping  from  the  surface 
are  strongly  colored,  and  the  more  such  multiple  reflec- 
tions there  are  the  more  pronounced  is  the  Selective 'color- 
ation due  to  reflection;  hence,  ordinary  colored  surfaces, 
from  which  diffuse  reflection  takes  place,  are  apt  to  take 
very  strongly  the  color^  of  the  pigment — more  strongly, 
perhaps,  than  a  casual  inspection  of  the  pigment  would 
suggest. 

Now,  as  we  shall  presently  see,  in  any  enclosed  space 
the  light  reflected  from  the  bounding  surfaces  is  a  very 


46  THE   ART   OF   ILLUMINATION. 

considerable  portion  of  the  whole,  and,  therefore,  if  these 
surfaces  are  colored,  the  general  illumination  is  strongly 
colored  also,  whatever  the  illuminant  may  be;  in  other 
words,  colored  surroundings  will  modify  the  color  of  the 
illumination  just  as  definitely  as  a  colored  shade  over  the 
source  of  light.  In  planning  the  general  color  tone  of  a 
room  to  be  illuminated,  it  must  be  remembered  that  if  the 
walls  are  strongly  colored  the  dominant  tone  of  the  illu- 
mination will  be  that  of  the  walls  rather  than  that  of  the 
light. 

An  interesting  corollary  resulting  from  Fig.  13  some- 
times appears  in  the  colors  of  certain  fabrics.  If  the 
surface  fibers  of  the  fabric  lie  in  one  general  direction  the 
light  reflected  from  that  fabric,  which  determines  its  visi- 
ble color,  follows  somewhat  the  same  laws  laid  down  for 
asymmetric  reflection,  discussed  in  the  case  of  Fig.  n. 

Light  falling  on  the  fabric  from  the  direction  toward 
which  the  surface  fibers  run  does  not  escape  without  pro- 
fuse multiple  reflection,  and  hence  takes  strongly  the  color 
of  the  pigment.  Light,  however,  falling  on  the  fabric 
reversely  to  the  direction  of  the  fibers  undergoes  much  less 
multiple  reflection,  and  is  likely  to  be  mixed  with  a  large 
amount  of  white  light  hardly  affected  by  pigment  at  all; 
hence,  the  curious  phenomenon  of  changeable  color  in 
fabrics — for  instance,  a  fine  purple  from  one  direction  of 
illumination  and  perhaps  very  light  pink  from  another. 

If,  in  addition  to  the  effects  resulting  from  an  admix- 
ture of  white  light  in  certain  directions  of  incidence,  one 
also  has  the  curiously  composite  colors  sometimes  found 
in  modern  dye  stuffs,  the  changeable  color  effects  may  be 
and  often  are  very  conspicuous ;  the  more  so,  since  in  such 
colors,  by  multiple  reflection,  or — what  amounts  to  the 
same  thing — by  more  or  less  complete  absorption  of  cer- 


REFLECTION   AND    DIFFUSION.  47 

tain  rays,  the  resultant  color  may  be  very  profoundly 
changed. 

Absorbing  media  sometimes  show  these  color  changes 
very  conspicuously;  as,  for  example,  chlorophyll,  the  green 
coloring  matter  of  leaves,  which  in  a  weak  solution  is 
green,  but  of  which  a  very  strong  solution  of  considerable 
thickness  transmits  only  the  dark  red  rays.  Similar  char- 
acteristics pertain  to  many  modern  dye  stuffs,  and  result, 
in  connection  with  the  composite  reflection  which  has  just 
been  explained,  in  some  very  extraordinary  and  very  beau- 
tiful effects. 

From  what  has  just  been  said  about  color  reflection  it  is 
obvious  enough  that  the  loss  in  intensity  in  a  reflected  ray 
may  be  very  considerable,  even  from  a  single  regular  re- 
flection under  quite  favorable  conditions.  Many  experi- 
ments have  been  made  to  find  the  absolute  loss  of  inten- 
sity due  to  reflection.  This  absolute  value  of  what  is  called 
the  coefficient  of  reflection — that  is  to  say,  the  ratio  be- 
tween the  intensities  of  the  incident  and  reflected  light — 
varies  very  widely  according  to  the  condition  of  the  re- 
flecting surface.  It  also,  in  case  the  surfaces  are  not  with- 
out selective  reflection  in  respect  to  color,  varies  notably 
with  the  color  of  the  incident  light. 

The  following  table  gives  a  collection  of  approximate 
results  derived  from  various  sources.  The  figures  show 
clearly  enough  the  uncertain  character  of  the  data : 


MATERIAL  COEFFICIENT 

OF  REFLECTION. 

Highly  polished  silver .92 

Mirrors  silvered  on  surface 70 — .85 

Highly  polished  brass , 70 — .75 

Highly  polished  copper 60 — .70 

Highly  polished  steel •   .60 

Speculum  metal 60 — .80 

Polished  gold 50— .55 

Burnished  copper 40—.  50 


48  THE   ART   OF   ILLUMINATION. 

The  losses  in  reflection  are  due  to  absorption  and  to  a 
certain  amount  of  diffuse  reflection  mixed  with  the  regular 
reflection.  The  above  figures  are  for  light  in  the  most  in- 
tense part  of  the  spectrum  and  for  rather  small  angles  of 
incidence.  For  large  angles  of  incidence — 85  degrees 
and  more — the  intensity  of  the  reflected  beam  is  materially 
diminished,  owing  probably  both  to  increase  in  absorption 
and  to  diffuse  reflection. 

Mirrors  silvered  with  amalgam  on  the  back,  and  various 
burnished  metals  sometimes  used  for  reflectors,  belong 
near  the  bottom  of  the  table  just  given.  Silver  is  dis- 
tinctly the  best  reflecting  surface;  under  very  favorable 
circumstances  the  coefficient  of  reflection  of  this  metal  is 
in  excess  of  .90.  A  very  little  tarnishing  of  the  sur- 
face results  in  increased  absorption  and  diffusion  and  a 
still  further  reduction  of  the  intensity  of  the  reflected  ray. 
The  values  of  these  coefficients  show  plainly  the  consider- 
able losses  which  may  be  incurred  in  using  reflectors  in 
connection  with  artificial  lighting. 

So  far  as  general  illumination  is  concerned,  the  light 
diffused  at  the  reflecting  surfaces  is  not  altogether  lost, 
but  that  absorbed  is  totally  useless.  In  the  case  of  or- 
dinary reflecting  surfaces  one  deals  with  a  mixture  of 
regular  and  diffused  reflection,  and  in  practical  illumina- 
tion the  .latter  is  generally  more  important  than  the  for- 
mer, for  it  determines  the  amount  of  light  which  reaches 
the  surface  to  be  illuminated  in  ways  other  than  direct 
radiation  from  the  illuminant. 

Obviously,  if  one  were  reading  a  book  in  a  room  com- 
pletely lined  with  mirrors,  the  effect  of  the  illumination 
upon  the  page  would  be  vastly  greater  than  that  received 
directly  from  the  source  of  light  itself.  On  the  other 
hand,  a  room  painted  black  throughout  would  give  very 


REFLECTION   AND    DIFFUSION.  49 

little  assistance  from  reflection,  and  the  illumination  upon 
the  page  would  be  practically  little  greater  than  that  re- 
ceived directly  from  the  lamp.  Between  these  limits  falls 
the  condition  of  ordinary  "illumination  in  enclosed  spaces. 
Generally  speaking,  there-is  very  material  assistance  from 
reflection  at  the  bounding  surfaces.  The  amount  of  such 
assistance  depends  directly  upon  the  coefficient  of  diffuse 


Fig.  14. — Asymmetric  Reflection  from  a  Fabric. 

reflection  of  the  various  surfaces  concerned,  varying  with 
the  color  and  texture  of  each. 

As  has  been  already  indicated,  diffuse  reflection  is 
rough,  heterogeneous,  regular  reflection,  more  or  less  com- 
plicated, according  to  the  texture  of  the  reflecting  surface, 
by  multiple  reflections  in  the  surface  before  the  ray  finally 
escapes,  and  therefore,  the  coefficients  of  diffuse  reflection 
are  not  so  widely  different  from  those  of  direct  reflection 
as  might  at  first  sight  appear  probable,  so  far  at  least  as 
the  total  luminous  effect  is  concerned. 

In  certain  kinds  of  diffuse  reflection  there  is  consider- 
able loss  from  absorption  as  well  as  from  multiple  reflec- 
tions. This  is  conspicuously  the  case  in  the  light  reflected 
from  fabrics,  where  there  is  not  only  reflection  from  the 
surface  fibers,  but  where  the  rays  before  escaping  are  more 
than  likely  to  have  to  traverse  some  of  them.  This  is 


5o  THE   ART   OF   ILLUMINATION. 

illustrated  in  a  rather  crude  but  typical  way  in  Fig.  14, 
which  gives  a  characteristic  case  of  asymmetric  reflection. 
We  may  suppose  that  the  beam  of  light  falls  upon  a  surface 
of  fabric  having  a  well-marked  nap.  In  the  cut  aa  is  the 
fabric  surface  composed  of  inclined  fibers  or  bunches  of 
fibers.  These  fibers,  although  colored,  are  more  or  less 
translucent  and  are  not  colored  uniformly  throughout 
their  substance.  Owing  to  their  direction,  rays  i,  2,  and 
3  get  completely  clear  of  the  surface  of  the  fabric  by  a 
single  reflection.  These  rays  are  but  slightly  colored,  be- 
cause of  the  comparatively  feeble  intensity  of  the  colora- 
tion of  the  individual  fibers,  which  have  a  strong  tendency 
to  reflect  white  light  from  the  shiny  surface. 

On  the  other  hand,  rays  4,  5,  and  6,  inclined  from  the 
other  direction,  are  several  times  reflected  before  clearing 
the  surface,  and  in  emerging  therefrom  have  to  pass 
through  the  bunches  of  translucent  fibers  that  form  the 
nap.  As  the  result  they  are  strongly  colored.  The 
amount  of  white  light  is  very  small  and  the  structure  of 
the  surface  has  produced  a  marked  changeable  coloration. 

In  reality,  of  course,  few  rays  actually  escape  on  a  single 
reflection,  and  those  striking  almost  in  line  with  the  direc- 
tion of  the  fibers,  as  4,  5,  and  6  in  the  figure,  may  be  re- 
flected many  times,  so  that  the  actual  effect  is  an  exaggera- 
tion of  that  illustrated. 

Moreover,  the  material  of  the  surface  fibers  exercises  a 
considerable  influence  on  the  amount  and  character  of  the 
selective  coloration.  Silk  is  especially  well  adapted  to 
show  changeable  color  effects,  since  its  fibers  can  be  made 
to  lie  more  uniformly  in  the  same  direction  than  the  fibers 
of  any  other  substance,  and  they  are  themselves  naturally 
lustrous,  so  as  to  be  capable  even  when  strongly  dyed  of 
reflecting,  particularly  at  large  angles  of  incidence,  a  very 


REFLECTION   AND    DIFFUSION.  51 

considerable  proportion  of  white  light.  Being  thus  lus- 
trous they  form  rather  good  reflecting  surfaces,  and  hence 
the  light  entangled  in  their  meshes  can  undergo  a  good 
many  reflections  without  losing  so  much  in  intensity  as  to 
dull  conspicuously  the  resulting  color  effect;  besides,  silk 
takes  dyes  much  more  easily  and  permanently  than  other 
fibers  and,  hence,  can  be  made  to  acquire  a  very  fine  color- 
ation. 

Wool  takes  dye  less  readify,  and  it  is  not  so  easy  to  give 
the  surface  fibers  a  definite  direction.  They  are,  however, 
quite  transparent  and  lustrous  enough  to  give  fine  rich 
colors.  Cotton  is  inferior  to 'both  silk  and  wool  in  these 
particulars;  hence,  the  phenomena  we  have  been  investi- 
gating are  seldom  marked  in  cotton  fabrics. 

In  velvet,  which  is  a  very  closely  woven  cut  pile  fabric, 
the  surface  fibers  forming  the  pile  stand  erect  and  very 
closely  packed  together.  It  is  difficult,  therefore,  for 
light  to  undergo  anything  except  a  very  complex  reflec- 
tion, and  practically  all  the  rays  which  come  from  the 
surface  have  penetrated  into  the  pile  and  acquired  a  strong 
coloration.  The  white  light  reflected  from  the  surface  of 
the  fibers  hardly  comes  into  play  at  all  except  at  large 
angles  of  incidence,  so  that  the  result  is  a  particularly 
strong,  rich  effect  from  the  dyes,  particularly  in  silk 
velvet. 

Cotton  velvet,  with  its  more  opaque  fibers,  seems  duller, 
and,  particularly  if  a  little  worn,  reflects  enough  light  from 
the  surface  of  the  pile  to  interfere  with  the  purity  and  in- 
tensity of  the  color.  Much  of  the  richness  in  color  of 
rough  colored  fabrics  and  surfaces  is  due  to  the  complete- 
ness of  the  multiple  reflections  on  the  dyed  fibers,  which 
produces  an  effect  quite  impossible  to  match  with  a  smooth 
surface  unless  dyed  with  the  most  vivid  pigments. 


52  THE   ART    OF   ILLUMINATION. 

In  practical  illumination  one  seldom  deals  with  fabrics 
to  any  considerable  extent,  but  almost  always  with  papered 
or  painted  surfaces.  These  are  generally  rather  smooth, 
except  in  the  case  of  certain  wall  papers  which  have  a  silky 
finish.  Smooth  papers  and  paint  give  a  very  considerable 
amount  of  surface  reflection  of  white  light,  in  spite  of  the 
pigments  with  which  they  may  be  colored.  The  diffusion 
from  them  is  very  regular,  except  for  this  surface  sheen, 
and  may  be  exceedingly  strong.  When  light  from  the 
radiant  point  falls  on  such  a  surface  it  produces  a  very 
wide  scattering  of  the  rays,  and  an  object  indirectly  illumi- 
nated therefore  receives  in  the  aggregate  a  very  large 
amount  of  light. 

A  great  many  experiments  have  been  tried  to  determine 
the  amount  of  this  diffuse  reflection  which  becomes  avail- 
able for  the  illumination  of  a  single  object.  The  general 
method  has  been  to  compare  the  light  received  directly 
from  the  illuminant  with  that  received  from  the  same 
illuminant  by  one  reflection  from  a  diffusing  surface. 

The  following  table  gives  an  aggregation  of  the  results 
obtained  by  several  experimenters,  mostly  from  colored 
papers. 

COEFFICIENT  OF 
MATERIAL  DIFFUSE  REFLECTION. 


blotting  paper  ................................  .........  82 

NWhite  cartridge  paper  ..................................  ,.     .80 

Ordinary  foolscap  ...........................................  70 

Chrome  yellow  paper  ...............  .........................  62 

Orange  paper  .............................................  50 

Plain  deal  (clean)..    ...  ...................................  45 

Yellow  wall  paper  .........................................  40 

^Yellow  painted  wall  (clean;  ...................................  40 

Light  pink  paper  ...........................................  36 

Yellow  cardboard   ..........................................  30 

Light  blue  cardboard  .......................................  25 

Brown  cardboard  ...........................................  20 

_^  Plain  deal  (dirty)  .................................  ..........      .20 

Yellow  painted  wall  (dirty)  .................................  20 

Emerald  green  paper  .....................................  ,     .18 

Dark  brown  paper  ...................  .  ............  ...........  13 


REFLECTION   AND   DIFFUSION.  53 

COEFFICIENT  OF 
MATERIAL.  DIFFUSE  REFLECTION. 

Vermilion  paper 12 

Blue-green  paper .12 

Cobalt  blue  paper 12 

"^  Black  paper 05 

Deep  chocolate  paper 04 

French  ultramarine  blue  paper 035 

\Black  cloth 012 

X  Black  velvet 004 

At  the  head  of  the  list  stands  white  blotting  paper, 
which  is  really  a  soft  mass  of  lustrous  white  fibers.  Its 
coefficient  of  reflection — .82 — is  comparable  with  the  co- 
efficient of  direct  reflection  from  a  mirror;  so  far,  at  least, 
as  lights  of  ordinary  intensity  are  concerned. 

White  cartridge  paper  is  a  good  second,  and  partakes  of 
the  same  general  characteristics. 

Of  the  colored  papers  only  the  yellows,  and  pink  so  light 
as  to  give  a  strong  reflection  of  white  light  from  the  un- 
colored  fibers,  have  coefficients  of  diffuse  reflection  of  any 
considerable  magnitude.  Very  light  colors  in  general 
diffuse  well  owing  to  the  uncolored  component  of  the  re- 
flected light,  but  of  those  at  all  strongly  colored  only  the 
yellows  are  conspicuously  luminous. 

Of  course,  all  of  the  papers  when  at  all  dirty  diffuse 
much  less  effectively  than  when  clean,  and  the  rough 
papers,  which  have  the  highest  coefficients  of  diffusion, 
are  particularly  likely  to  become  dirty. 

A  smooth,  clean  white  board  and  white  painted  surfaces 
generally  diffuse  pretty  well,  but  lose  rapidly  in  effective- 
ness as  they  become  soiled.  Greens,  reds,  and  browns,  in 
all  their  varieties,  have  low  coefficients,  and  it  is  worth 
noticing  that  deep  ultramarine  blue,  diffuses  even  less 
effectively  than  black  paper  coated  with  lamp-black,  which 
has  a  diffusion  of  .05  as  against  .035  for  the  blue.  Black 
cloth,  with  a  surface  rough  compared  with  the  black  paper, 


54  THE   ART   OF   ILLUMINATION. 

diffuses  very  much  less  light;  while  black  velvet — of  which 
the  structure  is,  as  just  explained,  particularly  adapted  to 
suppress  light — has  a  coefficient  of  diffusion  conspicu- 
ously less  than  any  of  the  others.  A  little  dust  upon  its 
surface,  however,  is  capable  of  reflecting  a  good  deal  of 
light. 

These  coefficients  of  diffusion  have  a  very  important 
bearing  on  the  illumination  of  interiors.  It  is  at  once  ob- 
vious that— except  in  the  case  of  a  white  interior  finish  or 
a  very  pale  shade  of  color — the  illumination  received  by 
any  object  is  not  very  greatly  strengthened  by  diffused 
light  from  the  walls.  All  of  the  strong  colors,  particu- 
larly if  very  dark,  cut  down  diffusion  to  a  relatively  small 
amount,  although  it  is  very  difficult  to  suppress  diffusion 
with  anything  like  completeness. 

One  of  the  standing  difficulties  in  photometric  work  is 
to  coat  the  walls  of  the  photometer  room  with  a  substance 
so  non-reflecting  as  not  to  interfere  with  the  measure- 
ments. Even  lamp-black  returns  as  diffused  light  one- 
twentieth  of  that  thrown  upon  it,  and  painting  with  any- 
thing less  lusterless  than  lamp-black  would  increase  the 
proportion  of  diffused  light  very  consideraby.  Walls 
painted  dead  black,  and  auxiliary  screens,  also  dead  black, 
to  cut  off  the  diffused  light  still  more,  are  the  means  gen- 
erally taken  to  prevent  the  interference  of  reflected  light 
with  the  accuracy  of  the  photometric  measurements. 

In  the  case  of  any  diffusing  surface,  or  any  reflecting 
surface  whatever,  for  that  matter,  a  second  reflection  has, 
at  least  approximately;  the  same  coefficient  of  reflection  as 
the  first,  so  that  for  trie  two  reflections  the  intensity  of  the 
beam  that  finally  escapes  is  that  of  the  incident  beam  mul- 
tiplied by  the  square  of  the  coefficient  of  diffusion,  and  so 
on  for  higher  powers. 


REFLECTION   AND    DIFFUSION.  55 

Inasmuch  as  in  any  enclosed  space  there  is  considerable 
cross-reflection  of  diffused  light,  the  difference  in  the  total 
amount  of  illumination  due  to  reflection  is  even  more  vari- 
able than  would  be  indicated  by  the  table  of  coefficients 
given;  for  while  the  amount  of  light  twice  diffused  from 
white  paper  or  paint  would  be  very  perceptible  in  the 
illumination,  that  twice  diffused  from  paper  of  a  dark 
color  would  be  comparatively  insignificant. 

The  color  of  the  walls,  therefore,  plays  a  most  impor- 
tant part  in  practical  illumination,  for  rooms  with  dark  or 
strongly-colored  walls  require  a  very  much  more  liberal 
use  of  illuminants  than  those  with  white  or  lightly-tinted 
walls.  The  difference  is  great  enough  to  be  a  considerable 
factor  in  the  economics  of  the  question  in  cases  where 
artistic  considerations  are  not  of  prime  importance.  The 
nature  and  amount  of  the  effect  of  the  bounding  surfaces 
on  illumination  will  be  discussed  in  connection  with  the 
general  consideration  of  interior  lighting. 


CHAPTER   IV. 

THE    MATERIALS    OF    ILLUMINATION — ILLUMINANTS 
OF     COMBUSTION. 

AT  root,  all  practical  illuminants  are  composed  of  solid 
particles,  usually  of  carbon,  brought  to  vivid  incandes- 
cence. We  may,  however,  divide  them  into  two  broad 
classes  according  as  the  incandescent  particles  are  heated 
by  their  own  combustion  or  by  extraneous  means.  The 
first  class,  therefore,  may  be  regarded  as  composed  of 
luminous  flames,  such  as  candles,  lamps,  ordinary  gas 
flames,  and  the  like,  while  the  second  consists  of  illumi- 
nants in  which  a  solid  is  rendered  incandescent,  it  is  true, 
but  not  by  means  of  its  own  combustion. 

The  second  class  thus  consists  of  such  illuminants  as 
mantle  gas  burners,  electric  incandescent  lamps,  and  the 
electric  arcs,  which  really  give  their  light  in  virtue  of  the 
intense  heating  of  the  tips  of  the  carbons  by  the  arc,  which 
in  itself  is  relatively  of  feeble  luminosity. 

Illumination  based  on  incandescent  gas,  phosphores- 
cence, and  the  like  is  in  a  very  early  experimental  stage, 
and  while  it  is  in  this  direction  that  we  must  look  for  in- 
creased efficiency  in  illumination,  nothing  of  practical  mo- 
ment has  yet  been  accomplished.  To  the  examination  of 
flame  illuminants,  then,  we  must  first  address  ourselves. 

They  are  interesting  as  being  the  earliest  sources  of 
artificial  light,  and  while  of  much  less  luminous  efficiency 
than  the  second  class  referred  to,  still  hold  their  own  in 


THE   MATERIALS   OF   ILLUMINATION.        57 

point  of  convenience,  portability,  and  ease  of  extreme  sub- 
division. 

We  have  no  means  of  knowing  the  earliest  sources  of 
artificial  light  as  distinguished  from  heat.  The  torch  of 
fat  wood  was  a  natural  development  from  the  fire  on  the 
hearth.  But  even  in  Homeric  times  there  is  clear  evi- 
dence of  fire  in  braziers  for  the  purpose  of  lighting,  and 
there  is  frequent  mention  of  torches.  The  rope  link  satu- 
rated with  pitch  or  bitumen  was  a  natural  growth  from  the 
pine  wood  torch,  and  was  later  elaborated  into  the  candle. 

It  is  clear  that  both  lamps  and  candles  date  far  back 
toward  prehistoric  times,  the  lamp  being  perhaps  a  little 
the  earlier  of  the  two.  At  the  very  dawn  of  ancient  civili- 
zation man  had  acquired  the  idea  of  soaking  up  animal 
or  vegetable  fats  into  a  porous  wick  and  burning  it  to  ob- 
tain light,  and  the  use  of  soft  fats  probably  preceded  the 
use  of  those  hard  enough  to  form  candles  conveniently. 

The  early  lamps  took  the  form  of  a  small  covered  basin 
or  jar  with  one  or  more  apertures  for  the  wick  and  a  sepa- 
rate aperture  for  filling.  They  were  made  of  metal  or  pot- 
tery, and  by  Roman  times  often  had  come  to  be  highly 
ornamented.  Fig.  15  shows  a  group  of  early  Roman 
lamps  of  common  pottery,  and  gives  a  clear  idea  of  what 
they  were.  They  rarely  held  more  than  one  or  two  gills, 
and  must  have  given  at  best  but  a  flickering  and  smoky 
light.  Fig.  1 6  shows  a  later  Roman  lamp  of  fine  work- 
manship in  bronze. 

In  very  early  times  almost  any  fatty  substance  that 
would  burn  was  utilized  for  light,  but  in  recent  centuries 
the  cruder  fats  have  largely  gone  out  of  use,  and  new 
materials  have  been  added  to  the  list.  It  would  be  a 
thankless  task  to  tabulate  the  properties  of  all  the  sub- 
stances which  have  been  burned  as  illuminants,  but  those 


58  THE   ART   OF   ILLUMINATION. 

in  practical  use  within  the  century  just  passed  may  for 
convenience  be  classified  about  as  follows : 


FLAME  ILLUMINANTS. 


FATS  AND  WAXES. 

Tallow  (stearin). 
Sperm  oil. 
Spermaceti. 
Lard  oil. 


FATS  AND  WAXES. 

Olive  oil. 
Whale  oil. 
Beeswax. 
Vegetable  waxes. 


The  true  fats  are  chemically  glycerides,  i.  e.,  combina- 
tions of  glycerin  with  the  so-called  fatty  acids,  mainly 


Fig.  15. — Early  Roman  Lamps. 

stearic,  oleic,  and  palmetic.  The  waxes  are  combinations 
of  allied  acids  with  bases  somewhat  akin  to  glycerin,  but 
of  far  more  complicated  composition.  Technically,  sper- 
maceti is  allied  to  the  waxes,  while  some  of  the  vegetable 
waxes  properly  belong  to  the  fats. 

All  these  substances,  solid  or  liquid,  animal  or  vege- 
table, are  very  rich  in  carbon.  They  are  composed  en- 
tirely of  carbon,  hydrogen,  and  oxygen,  and  as  a  class  have 


THE   MATERIALS   OF   ILLUMINATION, 


59 


about  the  following  percentage  composition  by  weight: 
Carbon,  76  to  82  per  cent.;  hydrogen,  n  to  13  per  cent; 
oxygen,  5  to  10  per  cent. 

They  are  all  natural  substances  which  merely  require  to 


Fig.  16. — Roman  Bronze  Lamp. 

go  through  a  process  of  separation  from  foreign  matter, 
and  sometimes  bleaching,  to  be  rendered  fit  for  use. 

An  exception  may  be  made  in  favor  of  "  stearin,"  which 
is  obtained  by  breaking  up  chemically  the  glycerides  of 
animal  fats  and  separating  the  fatty  acids  before  men- 
tioned from  the  glycerin.  The  oleic  acid,  in  which  liquid 
fats  are  rich,  is  also  gotten  rid  of  in  the  commercial  prepa- 
ration of  stearin  in  order  to  raise  the  melting  point  of  the 
product. 

In  a  separate  class  stand  the  artificial  "  burning  fluids  " 


60  THE   ART   OF   ILLUMINATION. 

used  considerably  toward  the  middle  of  the  century.  As 
they  are  entirely  out  of  use,  they  scarcely  deserve  particu- 
lar classification.  Their  base  was  usually  a  mixture  of 
wood  alcohol  and  turpentine  in  varying  proportions. 
From  its  great  volatility  such  a  compound  acted  almost 
like  a  gas  generator ;  the  flame  given  off  was  quite  steady 
and  brilliant,  with  much  less  tendency  to  smoke  than  the 
natural  oils,  but  the  "  burning  fluids  "  as  a  class  were  out- 
rageously dangerous  to  use,  and  fortunately  were  driven 
out  by  the  advent  of  petroleum  and  its  products. 

Petroleum,  which  occurs  in  one  form  or  another  at 
many  places  on  the  earth's  surface,  has  been  known  for 
many  centuries,  although  not  in  large  amounts  until 
recently.  Bitumen  is  often  mentioned  by  Herodotus  and 
other  early  writers,  and  in  Pliny's  time  mineral  oil  from 
Agrigentum  was  even  used  in  lamps. 

But  the  actual  use  of  petroleum  products  as  illuminants 
on  a  large  scale  dates  from  a  little  prior  to  1860,  when  the 
American  and  Russian  fields  were  developed  with  a  com- 
mon impulse.  Crude  petroleum  is  an  evil  smelling  liquid, 
varying  in  color  from  very  pale  yellow  to  almost  black, 
and  in  specific  gravity  from  0.77  to  i.oo,  ranging  com- 
monly from  0.80  to  0.90. 

Chemically  it  is  composed  essentially  of  carbon  and 
hydrogen,  its  average  percentage  composition  being  about 
as  follows:  carbon,  85  per  cent.;  hydrogen,  15  per  cent. 
It  is  composed  in  the  main  of  a  mixture  of  the  so-called 
paraffin  hydrocarbons,  having  the  general  formula 
Cn  H.jn_j_3,  and  the  members  of  this  series  found  in 
ordinary  American  petroleum  vary  from  methane  (  CH4) 
to  pentadecane  (C1B  H32),  and  beyond  to  solid  hydro- 
carbons still  more  complicated. 

To  fit  petroleum  for  use  as  an  illuminant,  these  com- 


THE  MATERIALS   OF   ILLUMINATION.        61 


ponent  parts  have  to  be  sorted  out,  so  that  the  oil  for 
burning  shall  neither  be  so  volatile  as  to  have  a  danger- 
ously low  flashing  point  nor  so  stable  as  not  to  burn 
clearly  and  freely. 

This  sorting  is  done  by  fractional  distillation.  The 
following  table  gives  a  general  idea  of  the  products  ar- 
ranged according  to  their  densities : 


SUBSTANCE. 

\  Cymogene, 
Petroleum  ether. . .  j  Rhigoline, 

'  Gasoline, 

(  Benzine  naphtha, 
Petroleum  spirit. .  )  Naphtha, 

(  Benzine, 

Kerosene J  Kerosene  of  va- 

1  rious  grades, 

Oils. .  . .  \  Lubricating  oils 

f  of  various  grades. 


USE. 


Small. 


Solids 


(  Vas< 

ids ] 

(  Pars 


Vaseline, 
I'araffin, 


Gas,  explosion  engines. 
Gas  lamps,  engines. 
Cleaning,  engines. 
Varnish,  etc. 

Illumination. 


Lubrication. 

Emollient. 
Candles,  insulation, 
waterproofing,  etc. 


"  Petroleum  ether  "  and  "  petroleum  spirit  "  find  little 
use  in  illumination,  for  they  are  so  inflammable  as  to  be 
highly  dangerous,  and  form  violently  explosive  mixtures 
with  air  at  ordinary  temperatures. 

Kerosene  should  be  colorless,  without  a.  very  penetrat- 
ing odor,  which  indicates  too  great  volatility,  and  should 
not  give  off  inflammable  vapor  below  a  temperature  of 
120°  F.,  or,  better  still,  below  140°  F.  to  150°  F.  Oils  of 
the  latter  grades  are  pretty  safe  to  use,  and  are  always  to 
be  preferred  to  those  more  volatile.  The  yield  of  kero- 
sene from  crude  oil  varies  from  place  to  place,  but  with 
good  American  oil  runs  as  high  as  50  to  75  per  cent. 

Paraffin  is  sometimes  used  unmixed  for  making  candles, 


62  THE   ART   OF   ILLUMINATION. 

but  is  preferably  mixed  with  other  substances,  like  stearin, 
to  give  it  a  higher  melting  point. 

Having  thus  casually  looked  over  the  materials  burned 
in  candles  and  lamps,  the  results  may  properly  be  con- 
sidered. 

Candles. — These  are  made  usually  of  stearin,  paraffin, 
wax,  or  mixtures  of  the  two  first  named.  They  are 
molded  hot  in  automatic  machines,  and,  as  usually  sup- 
plied in  this  country,  are  made  in  weights  of  4,  6,  and  12 
to  the  pound.  Spermaceti  candles  are  also  made,  but  are 
little  used  except  for  a  standard  of  light.  The  English 
standard  candle  is  of  spermaceti,  weighing  one-sixth  of  a 
pound  and  burning  at  the  rate  of  120  grains  per  hour. 

Commercial  candles  give  approximately  one  candle- 
power,  sometimes  rather  more,  and  burn  generally  from 
no  to  130  grains  per  hour.  As  candles  average  from 
15  to  1 8  cents  per  pound,  the  cost  of  one  candle-hour  from 
this  source  amounts  to  about  0.25  cent  to  0.30  cent.  This 
is  obviously  relatively  very  expensive,  although  it  must 
not  be  forgotten  that  candles  subdivide  the  light  so  effect- 
ively that  for  many  purposes  16  lighted  candles  are  very 
much  more  effective  in  producing  illumination  than  a  gas 
flame  or  incandescent  lamp  of  16  candle  power. 

The  present  function  of  candles  in  illumination  is  con- 
fined to  their  use  as  portable  lights,  for  which,  on  the  score 
of  safety,  they  are  far  preferable  to  kerosene  lamps,  and  to 
cases  in  which,  for  artistic  purposes,  thorough  sub- 
division of  the  light  is  desirable.  Where  only  a  small 
amount  of  general  light  is  needed,  candles  give  a  most 
pleasing  effect  and  are,  moreover,  cleanly  and  odorless. 

In  efficiency  candles  leave  much  to  be  desired.  For, 
taking  the  ordinary  stearin  candle  as  a  type,  it  requires  in 
dynamical  units  90  watts  per  candle-power,  consumes  per 


THE   MATERIALS   OF   ILLUMINATION.        63 

hour  the  oxygen  contained  in  4.5  cu.  ft.  of  air,  and  gives 
off  about  0.6  cu.  ft.  of  carbonic  acid  gas.  In  these  re- 
spects the  candle  is  inferior  to  the  ordinary  lamp,  and  still 
more  inferior  to  gas  or  electric  lights.  Nevertheless,  it  is 
oftentimes  a  most  convenient  illuminant. 
r~Qil  Lamps. — Oils  other  than  kerosene  are  used  in  this 
country  only  to  a  very  slight  extent,  the  latter  having 
driven  out  its  competitors.  Sperm  oil  and,  abroad,  colza 
oil  (obtained  from  rape  seed)  are  valued  as  safe  and  re- 
liable illuminants  for  lighthouses,  and  in  some  parts  of  the 
Continent  olive  oil  is  used  in  lamps,  as  it  has  been  from 
time  immemorial.^? 

Here,  kerosene  is  still  the  general  illuminant  outside  of 
the  cities  and  larger  towns.  It  has  the  merits  of  being 
cheap  (on  the  average  12  cents  to  15  cents  per  gallon  in 
recent  years),  safe,  if  of  the  best  quality,  and  of  giving, 
when  properly  burned,  a  very  steady  and  brilliant  light. 
UAH  oils  require  a  liberal  supply  of  air  for  their  combus- 
tion, particularly  the  heavier  oils,  and  many  ingenious 
forms  of  lamp  have  been  devised  to  meet  the  requirements. 
On  the  whole,  the  most  successful  are  on  the  Argand  prin- 
ciple, using  a  circular  wick  with  air  supply  both  within  and 
without,  although  some  of  the  double  flat  wick  burners 
are  admirable  in  their  results.  A  typical  lamp^the  fa- 
miliar "  Rochester,"  is  shown  in  Fig.  17,  which  sufficiently 
shows  the  principle  involved.  In  kerosene  lamps  the 
capillary  action  of  the  wick  affords  an  ample  supply  of  oil, 
but  with  some  other  oils  it  has  proved  advantageous  to 
provide  a  forced  supply.  The  so-called  "  student  lamp," 
with  its  oil  reservoir,  is  the  survival  of  an  early  form  of 
Argand  burner  designed  to  burn  whale  oil.  In  other  in- 
stances clock-work  is  employed  to  pump  the  oil,  and  some- 
times a  forced  air  supply  is  used. 


TY  ) 


64  THE   ART   OF   ILLUMINATION. 

Kerosene  lamps  usually  are  designed  to  give  from  10 
to  20  candle-power,  and  occasionally  more,  special  lamps 
giving  even  up  to  50  or  60  candle-power.  The  consump- 
tion of  oil  is  generally  from  50  to  60  grains  per  hour  per 


Fig.  17. — "Rochester"  Kerosene  Burner. 

candle-power.  As  kerosene  weighs  about  6.6  pounds  per 
gallon,  the  light  obtained  is  in  the  neighborhood  of  800 
candle-hours  per  gallon. 

This  brings  the  cost  of  the  candle-hour  down  to  about 
0.018  cent,  taking  the  oil  at  15  cents  per  gallon.  No 
illuminants  save  arc  lights  and  mantle  burners  with 
cheap  gas  can  compare  with  it  in  point  of  economy^ 

A  very  interesting  and  valuable  application  of  oil  light- 
ing is  found  in  the  so-called  "  Lucigen  "  torch  and  several 
kindred  devices.  The  oil,  generally  one  of  the  heavier 
petroleum  products,  is  carried  under  air  pressure  in  a  good- 
sized  portable  reservoir,  and  the  oil  is  led,  with  the  com- 
pressed air  highly  heated  by  its  passage  through  the  appa- 
ratus, to  an  atomizing  nozzle,  from  which  it  is  thrown  out 


THE  MATERIALS   OF   ILLUMINATION.         65 


Fig.  1 8. — "  Lucigen  "  Torch. 

in  a  very  fine  spray,  and  is  instantly  vaporized  and  burned 
under  highly  efficient  conditions. 

These  "  Lucigen  "  torches  give  nearly  2000  candle- 


66  THE   ART   OF   ILLUMINATION. 

power  on  a  consumption  of  about  two  gallons  of  oil  per 
hour,  burning  with  a  tremendous  flaring  flame  three  feet 
or  more  in  length  and  six  or  eight  inches  in  diameter. 
They  are  very  useful  for  lighting  excavations  and  other 
rough  works  for  night  labor,  being  powerful,  portable,  and 
cheap  to  operate.  Fig.  18  gives  an  excellent  idea  of  this 
apparatus  in  a  common  form.  Such  a  light  is  only  suited 
to  outdoor  work,  but  it  forms  an  interesting  transitional 
step  toward  the  air-gas  illuminants  which  have  come  into 
considerable  use  for  lighting  where  service  mains  for  gas 
or  electricity  are  not  available,  or  where  the  conditions  call 
for  special  economy. 

^Air  Gas. — It  has  been  known  for  seventy  years  or  more 
that  the  vapor  of  volatile  hydrocarbons  could  be  used  to 
enrich  poor  coal  gas,  and  that  even  air  charged  with  a 
large  amount  of  such  vapor  was  a  pretty  good  illuminant. 
Of  late  years  this  has  resulted5  in  the  considerable  use  of 
"carbureters,"  which  saturate  air  with  hydrocarbon 
vapor,  making  a  mixture  too  rich  to  be  in  itself  explosive 
and  possessing  good  illuminating  properties  when  burned 
as  gas  in  the  ordinary  way.  The  usual  basis  of  opera- 
tions is  commercial  gasoline,  which  consists  of  a  mixture 
of  the  more  volatile  paraffin  hydrocarbons,  chiefly  pen- 
tane,  hexane  and  iso-hexane. 

The  process  of  gas-making  is  very  simple,  consisting 
merely  of  charging  air  with  the  gasoline  vapor.  Fig.  19 
shows  in  section  a  typical  air-gas  machine.  It  consists  of 
a  large  metal  tank  holding  a  supply  of  gasoline,  a  carburet- 
ing chamber  of  flat  trays  over  which  a  gasoline  supply 
trickles,  a  fan  to  keep  up  the  air  supply,  and  a  little  gas 
reservoir  in  which  the  pressure  is  regulated  and  from 
which  the  gas  is  piped.  The  fan  is  driven  by  heavy 
weights,  wound  up  at  suitable  intervals. 


THE  MATERIALS    OF   ILLUMINATION.        67 

The  whole  gas  machine  is  usually  put  in  an  under- 
ground chamber,  both  for  security  from  fire  and  to  aid  in 
maintaining  a  steady  temperature.  About  six  gallons  of 
gasoline  are  required  per  1000  cu.  ft.  of  air,  and  the  result 


Fig.  19. — Gasoline  Gas  Machine. 

is  a  gas  of  very  fair  illuminating  power,  rather  better 
than  ordinary  city  gas. 

The  cost  of  this  air  gas  is  very  moderate,  but  on  account 
of  the  cost  of  plant  and  some  extra  labor,  it  is  materially 
greater  than  the  cost  of  direct  lighting  by  kerosene  lamps. 
It  is  a  means  of  lighting  very  useful  for  country  houses 
and  other  places  far  from  gas  or  electric  supply  companies. 


68  THE   ART   OF   ILLUMINATION. 

The  principal  difficulty  is  the  variation  of  the  richness  of 
the  mixture  with  the  temperature,  owing  to  change  in  the 
volatility  of  the  gasoline,  a  fault  which  is  very  difficult  to 
overcome.  At  low  temperatures  there  is  a  tendency  to 
carburet  insufficiently  and  to  condense  liquid  in  the  cold 
pipes.  The  gas  obtained  from  these  machines  is  burned 
in  the  ordinary  way,  although  burners  especially  adapted 
for  it  are  extensively  employed.  Recently  such  gas  has 
been  considerably  used  with  mantle  burners,  obtaining 
thus  a  very  economical  result. 

Coal  Gas. — In  commercial  use  for  three-quarters  of  a 
century,  coal  gas  was,  until  about  twenty  years  ago,  the 
chief  practical  illuminant.  Little  need  here  be  said  of  its 
manufacture,  which  is  a  department  of  technology  quite 
by  itself,  other  than  that  the  gas  is  obtained  from  the  de- 
structive distillation  of  rich  coals  enclosed  in  retorts,  from 
which  it  is  drawn  through  purifying  apparatus  and  re- 
ceived in  the  great  gasometers  familiar  on  the  outskirts  of 
every  city. 

The  yield  of  gas  is  about  10,000  cu.  ft.  per  ton  of  coal  of 
good  quality.  The  resulting  gas  consists  mainly  of 
hydrogen  and  of  methane  (CH4)  with  small  amounts  of 
other  gases,  the  composition  varying  very  widely  in  de- 
tails while  preserving  the  same  general  characteristics. 
A  typical  analysis  of  standard  coal  gas  giving  16  to  17 
candle-power  for  a  burner  consuming  5  cu.  ft.  per  hour 
would  be  about  as  follows : 

Hydrogen 53.0 

Paraffin  hydrocarbons   33.0 

Other  hydrocarbons 3. 5 

Carbon  monoxide 5.5 

Carbon    dioxide , 0.6 

Nitrogen 4.2 

Oxygen 0.2 


100. 0 


THE  MATERIALS   OF   ILLUMINATION.        69 

Ammonia  compounds,  carbon  dioxide,  and  sulphur 
compounds  are  the  principal  impurities  which  have  to  be 
removed.  Traces  of  these  and  of  moisture  are  often 
found  in  commercial  gas. 

In  point  of  fact,  at  the  present  time  but  a  small  propor- 
tion of  the  illuminating  gas  used  in  this  country  is  un- 
mixed coal  gas,  such  as  might  show  the  analysis  just 
given.  Most  of  it  is  water  gas,  or  a  mixture  of  coal  gas 
and  water  gas.  Water  gas  is  produced  by  the  simple 
process  of  passing  steam  through  a  mass  of  incandescent 
coal  or  coke,  and  thus  breaking  up  the  steam  into  hy- 
drogen and  oxygen,  which  latter  unites  with  the  carbon 
of  the  coal,  forming  carbon  monoxide. 

At  moderate  temperatures  considerable  carbon  dioxide 
would  be  formed,  but,  as  this  is  worse  than  useless  for 
burning  purposes,  the  heat  is  always  carried  high  enough 
to  insure  the  formation  of  the  monoxide.  The  hypo- 
thetical chemical  equation  is  : 


The  reaction  is  never  clean  in  so  complete  a  sense  as 
this,  some  CO3  always  being  formed.  This  water  gas  as 
thus  formed  is  useless  as  an  illuminant,  and  requires  to 
be  enriched  by  admixture  of  light-producing  hydro- 
carbons —  carbureted,  in  other  words.  This  is  done  by 
treating  it  to  a  spray  of  petroleum  in  some  form,  and  at 
once  passing  the  mixture  through  a  superheater,  which 
breaks  down  the  heavier  hydrocarbons  and  renders  the 
mixture  stable. 

There  are  many  modifications  of  this  system  worked  on 
the  same  general  lines.  The  enriching  is  carried  to  the 
extent  necessary  to  meet  the  legal  requirements,  usually 
producing  gas  of  15  to  20  candle-power  for  a  5-ft.  jet. 


7o  THE   ART   OF   ILLUMINATION. 

A  typical  analysis  of  the  water  gas  after  enriching  would 
show  about  the  following  by  volume : 

Hydrogen 34-O 

Methane 1 5 -O 

Enriching  hydrocarbons 12.5 

Carbon  monoxide 33- ° 

Oxygen,  nitrogen,  etc 5-5 


The  latter  part  of  the  enriching  process,  i.  e.,  superheat- 
ing and  breaking  up  the  heavy  hydrocarbons  while  in  the 
form  of  vapor,  is  substantially  that  used  in  making 
Pintsch  and  allied  varieties  of  oil  gas,  so  that  commercial 
water  gas  may  be  regarded  as  a  mixture  of  water  gas  and 
oil  gas. 

Water  gas,  when  properly  enriched,  is  fully  the  equiva- 
lent of  coal  gas  for  illuminating  purposes.  The  main  dif- 
ference between  them  is  the  very  large  proportion  of  car- 
bon monoxide  in  the  water  gas,  which  adds  greatly  to  the 
danger  of  leaks. 

For  this  carbon  monoxide  is  an  active  poison,  not  kill- 
ing merely  by  asphyxia,  but  by  a  well-defined  toxic  action 
peculiar  to  itself.  Hence  persons  overcome  by  water  gas 
very  frequently  die  under  circumstances  which,  if  coal  gas 
were  concerned,  would  result  only  in  temporary  insensi- 
bility. As  the  enriched  water  gas  is  cheaper  than  coal 
gas,  however,  the  gas  companies,  maintaining,  with  some 
justice,  that  gas  is  not  furnished  for  breathing  purposes, 
supply  it  unhesitatingly — sometimes  openly,  sometimes 
without  advertising  the  fact. 

Very  commonly  so-called  coal  gases  contain  enriched 
water  gas  to  bring  up  their  illuminating  power.  In  these 
cases  the  carbon  monoxide  is  in  much  less  proportion, 
perhaps  only  12  to  15  per  cent. 

It  is  often  stated  that  water  gas  is  doubly  dangerous 


THE   MATERIALS   OF   ILLUMINATION.         ?i 

from  its  lack  of  odor.  The  unenriched  gas  is  practically 
odorless,  but  when  enriched  the  odor,  while  less  penetrat- 
ing than  that  of  coal  gas,  is  sufficiently  distinctive  to  make 
a  leak  easily  perceptible. 

L£ras  burners  for  ordinary  illuminating  gas  are  of  three 
general  types :  flat  flame,  Argand,  and  regenerative.  The 
first  named  is  the  most  common  and  least  efficient  form. 
It  consists  of  two  general  varieties,  known  respectively  as 
the  "  fishtail  "  and  "  bat's-wing."  The  former  has  a  con- 
cave tip,  usually  of  steatite  or  similar  material,  containing 
two  minute  round  apertures,  so  inclined  that  the  two  little 
jets  meet  and  flatten  out  crosswise  into  a  wide  flame. 
This  form  is  now  relatively  little  used  save  in  dealing 
with  some  special  kinds  of  gas. 

The  bat's-wing  burner,  with  a  dome-shaped  tip,  having 
a  narrow  slit  for  the  gas  jet,  is  the  usual  form  employed 
with  ordinary  gas.  Flat-flame  burners  work  badly  in 
point  of  efficiency  unless  of  fairly  large  size.  On  ordinary 
gas  of  14  to  i7-cp  nominal  value  on  a  5-ft.  burner, 
burners  taking  less  than  about  4  cubic  ft.  per  hour  are 
decidedly  inefficient.  A  4-ft.  burner  will  give  about  2.5 
candles  per  foot,  while  a  5-ft.  burner  will  give  2.75  to  3 
candle-power  per  foot. 

The  Argand  burners  give  considerably  better  results, 
their  flames  being  inclosed  and  protected  from  draughts 
by  a  chimney;  and  the  air  supply  being  good  the  tempera- 
ture of  the  flame  is  high  and  the  light  is  whiter  than  in  the 
flat-flame  burners.  The  principle  is  familiar,  the  wick  of 
the  Argand  oil  lamp  being  replaced  in  the  gas  burner  by  a 
hollow  ring  of  steatite  connected  with  the  supply,  and 
perforated  with  tiny  jet  holes  around  the  upper  edge. 
Fig.  20  shows  in  section  an  Argand  burner  (Suez's)  of  a 
standard  make  used  in  testing  London  gas.  This  burner 


7*  THE   ART   OF   ILLUMINATION. 

uses  5  cubic  feet  per  hour,  and  the  annular  chamber  has  24 
holes,  each  0.045"  m  diameter.  The  efficiency  is  a  little 
better  than  that  of  the  flat-flame  burners,  running,  on  good 


Fig.  20. — Section  of  Argand  Gas  Burner. 

gas,  from  3  to  3.5  candle-power  per  foot.  The  London 
legal  standard  gas  is  of  16  candle-power  in  this  5-ft. 
burner. 

On  rich  gas  the  flat-flame  burners,  particularly  the  fish- 
tail, work  better  than  the  Argand,  the  fishtail  being  better 
on  very  rich  gas  than  is  the  bat's-wing  form.  With 


THE  MATERIALS   OF   ILLUMINATION.        73 

ordinary  qualities  of  gas,  however,  the  Argand  burner  is 
vastly  more  satisfactory  than  the  flat  flames/^ 

For  very  powerful   lights  the  so-called  regenerative 
burners  are  generally  preferred.     These  are  based  on  the 


Fig.  21. — Wenham  Regenerative  Burner. 

general  principle  of  heating  both  the  gas  and  the  air  fur- 
nished for  its  combustion  prior  to  their  reaching  the 
flame.  The  burner  proper  is  something  like,  an  inverted 
Argand,  so  arranged  as  to  furnish  a  circular  sheet  of  flame 
convex  downward,  and  with,  of  course,  a  central  cusp. 
Directly  above  the  burner  and  strongly  heated  by  the 
flame,  are  the  air  and  gas  passages. 

Fig.  21  shows  in  section  the  Wenham  burner  of  this 


74 


THE   ART   OF   ILLUMINATION. 


class.  The  arrows  show  the  course  of  the  air  and  the  gas, 
the  latter  being  burned  just  below  the  iron  regenerative 
chamber  and  the  products  of  combustion  passing  upward 


Fig.  22. — Siemens  Regenerative  Gas  Burner. 


through  the  upper  shell  of  the  lamp,  and  preferably  to  a 
ventilating  flue.  The  globe  below  prevents  the  access  of 
cold  air,  and  the  annular  porcelain  reflector  surrounding 
the  exit  flue  turns  downward  some  useful  light. 

The  Siemens  regenerative  burner,  shown  in  Fig.  22,  is 


THE  MATERIALS   OF   ILLUMINATION.        75 

arranged  upon  a  similar  plan  and  gives  much  the  same 
effect.  The  regenerative  burners  of  this  class  give  a  very 
brilliant  yellow-white  light  with  a  generally  hemispherical 
distribution  downward.  They  work  best  and  most  eco- 
nomically in  the  larger  sizes,  100  to  200  candle-power, 
and  must  be  placed  near  the  ceiling  to  take  the  best  advan- 
tage of  their  usual  distribution  of  light. 

With  gas  of  about  i6-cp  standard  these  regenerative 
burners  consume  only  about  i  cubic  foot  per  hour  for  5  to 
7  candle-power.  They  are  thus  nearly  twice  as  economi- 
cal as  the  best  Argand  burners.  Their  chief  disadvantage 
lies  in  the  fact  that  to  get  this  economy  very  powerful 
burners  must  be  used,  of  a  size  not  always  conveniently 
applicable. 

From  such  a  powerful  center  of  light  a  large  amount  of 
heat  is  thrown  off,  obviously  less  per  candle-power  of  light 
than  in  other  gas  burners,  but,  in  the  aggregate,  large. 
Regenerative  burners  are  well  suited,  however,  to  the 
illumination  of  large  spaces,  although  at  the  present  time 
the  greater  economy  of  the  mantle  burner  has  rather 
pushed  the  regenerative  class  into  the  background.  Their 
light,  nevertheless,  is  of  a  very  much  more  desirable  color 
than  that  given  by  the  mantle  burners. 

The  most  recent  and  in  some  respects  mo^t  important 
addition  to  the  list  of  flame  illuminants  is  acetylene.  This 
gas  is  a  hydrocarbon  having  the  formula  C2  H2,  which  has 
been  well  known  to  chemists  for  many  years,  but  which 
until  recently  has  not  been  preparable  by  any  convenient 
commercial  process.  It  is  a  rather  heavy  gas,  of  evil  odor, 
generally  somewhat  reminiscent  of  garlic/  and,  being  very 
rich  in  carbon  uncombined  with  oxygen  (nearly  93  per 
cent,  by  weight)  It  burns  very  brilliantly  when  properly 
supplied  with  air.  Its  flame  is  intensely  bright,  nearly 


76  THE   ART   OF   ILLUMINATION. 

white  in  color,  and  for  the  light  given  it  vitiates  the  air  in 
comparatively  small  degree. 

Acetylene  is  made  in  practice  from  calcic  carbide, 
Ca  C2,  a  chemical  product  prepared  by  subjecting  a  mix- 
ture of  powdered  lime  and  carbon  (coke)  to  the  heat  of 
the  electric  furnace.  By  this  means  it  can  be  prepared 
readily  in  quantity  at  moderate  cost.  The  acetylene  is 
made  from  the  calcic  carbide  by  treating  it  with  water, 
lime  and  acetylene  being  the  results  of  the  reaction,  which, 
in  chemical  terms,  is  as  follows : 

Ca  Ca  +  2  H2  O  =  Ca  (OH)a  -f  C,  H2. 

Commercial  calcic  carbide  is  far  from  being  chemically 
pure,  so  that  the  acetylene  prepared  from  it  contains  vari- 
ous impurities,  and  is  neither  in  quantity  nor  quality  just 
what  the  equation  would  indicate.  The  carbide  is  ex- 
tremely hygroscopic,  and  hence  not  very  easy  to  transport 
or  keep,  and  the  upshot  of  this  property  and  the  inherent 
impurities  is  that  the  practical  yield  of  acetylene  is  only 
about  4.5  to  5.0  cubic  feet  per  pound  of  carbide,  4.75  cubic 
feet  being  an  extremely  good  average  unless  the  work  is 
on  a  very  large  scale,  though  4.5  cubic  feet  is  the  more 
usual  yield.  In  theory  the  yield  should  be  nearly  5.5 
cubic  feet  Qer  pound. 

The  gaseous  impurities  are  quite  varied  and  by  no 
means  uniform  in  amount  or  nature,  but  the  most  objec- 
tionable ones  may  be  removed  by  passing  the  gas  in  fine 
bubbles  through  water.  If  the  gas  is  being  prepared  on 
a  large  scale  it  can  readily  be  purified. 

Acetylene  has  the  disadvantage  of  being  somewhat  un- 
stable. It  forms  direct  compounds  with  certain  metals, 
notably  copper,  these  compounds  being  known  as  acety- 
lides,  and  being  themselves  so  unstable  as  to  be  easily  ex- 


THE  MATERIALS   OF  ILLUMINATION.        77 

plosive.     Acetylene  should  be  therefore  kept  out  of  con- 
tact with  copper  in  storage,  and  even  in  fixtures. 

The  gas  itself  is  easily  dissociated  with  evolution  of 
heat  into  carbon  and  hydrogen,  and  hence  may  be  inher- 
ently explosive  under  certain  conditions,  fortunately  not 
common. 

At  atmospheric  pressure,  or  at  such  small  increased 
pressures  as  are  employed  in  the  commercial  distribution 
of  gas,  acetylene,  unmixed  with  air,  cannot  be  exploded 
by  any  means  ordinarily  at  hand. 

Above  a  pressure  of  about  two  atmospheres  acetylene 
is  readily  explosive  from  high  heat  and  from  a  spark  or 
flame,  and  grows  steadily  in  explosive  violence  as  the 
initial  pressure  rises,  until  when  liquefied  it  detonates  with 
tremendous  power  if  ignited.  At  ordinary  temperatures  it 
can  be  liquefied  at  a  pressure  of  about  80  atmospheres, 
and  it  has  been  proposed  to  transport  and  store  it  in 
liquid  form.  But,  although  even  when  liquefied  it  will  not 
explode  from  mechanical  shgck  alone,  it  is  in  this  condi- 
tion an  explosive  of  the  same  order  of  violence  as  gun- 
cotton  or  nitro-glycerine,  and  should  be  treated  as  such. 

Mixtures  of  acetylene  and  air  explode  violently,  just  as 
do  mixtures  of  illuminating  gas  and  air.  The  former  be- 
gin to  explode  rather  than  merely  burn,  when  the  mix- 
ture contains  about  one  volume  of  acetylene  to  three  of 
air,  detonate  very  violently  with  about  nine  volumes  of 
air,  and  cease  to  explode  with  about  twenty  volumes  of 
air. 

Ordinary  coal  gas  begins  to  explode  when  mixed  with 
three  volumes  of  air,  reaches  a  maximum  of  violence  with 
about  five  to  six  volumes,  and  ceases  to  explode  with 
eleven  volumes.  Of  the  two  gases,  the  acetylene  is  rather 
the  more  violently  explosive  when  mixed  with  air,  and  it 


7  8  THE   ART   OF   ILLUMINATION. 

becomes  explosive  while  the  mixture  is  much  leaner.  The 
difference  is  not  of  great  practical  moment,  however,  ex- 
cept as  acetylene  generators,  being  easily  operated,  are 
likely  to  get  into  unskillful  hands.  This  fact  has  already 
resulted  in  many  disastrous  explosions. 

As  regards  its  poisonous  properties,  acetylene  seems  to 
be  somewhat  less  dangerous  than  coal  gas  and  very  much 
less  dangerous  than  water  gas.  Properly  speaking,  acety- 
lene is  very  feebly  poisonous  when  pure,  and  has  such  an 
outrageous  smell  when  slightly  impure  that  the  slightest 
leak  attracts  attention.  Some  early  experiments  showed 
highly  toxic  properties,  but  these  have  not  been  fully  con- 
firmed, and  may  have  been  due  to  impurities  in  the  gas — 
possibly  to  phosphine,  which  is  a  violent  poison. 

The  calcic  carbide  from  which  the  acetylene  is  pre- 
pared is  so  hygroscopic  and  gives  off  the  gas  so  freely  that 
it  has  to  be  stored  with  great  care  on  account  of  possible 
danger  from  fire.  Fire  underwriters  are  generally  united 
in  forbidding  entirely  the  use  or  storage  of  liquid  or  com- 
pressed acetylene,  or  the  storage  of  any  but  trivial  amounts 
of  calcic  carbide  (a  few  pounds)  except  in  detached  fire- 
proof buildings. 

Acetylene  is,  when  properly  burned,  a  magnificent 
illuminant.  It  will  not  work  in  ordinary  burners,  for  un- 
less very  liberally  supplied  with  air  it  is  so  rich  in  carbon 
as  to  burn  with  a  smoky  flame  and  a  deposit  of  soot.  It 
must  actually  be  mixed  with  air  at  the  burner  in  order  to 
be  properly  consumed.  When  so  utilized  its  illuminating 
power  is  very  great.  The  various  experiments  are  not 
closely  concordant,  but  they  unite  in  indicating  an  illumi- 
nating power  of  35  to  45  candle-hours  per  cubic  foot,  ac- 
cording to  the  capacity  of  the  burner,  the  larger  burners, 
as  usual,  working  the  more  economically. 


THE   MATERIALS   OF   ILLUMINATION.        79 

This  means  that  the  acetylene  has  nearly  fifteen  times 
the  illuminating  power  of  a  good  quality  of  ordinary 
illuminating  gas  when  burned  in  ordinary  burners.  It 
will,  consequently,  give  about  eight  to  ten  times  more  light 
per  cubic  foot  than  gas  in  a  regenerative  burner,  and,  it 
may  be  mentioned,  about  three  to  four  times  more  light 
than  gas  in  a  mantle  (Welsbach)  burner. 

Fig.  23  shows  a  common  standard  form  of  acetylene 
burner,  intended  to  consume  about  0.5  cubic  foot  per  hour. 


Fig.  23. — Acetylene  Burner. 

It  is  a  duplex  form  akin  in  its  production  of  flame  to  a 
common  fishtail.  Each  of  the  two  burners  is  formed  with 
a  lava  tip  having  a  slight  constriction  close  to  its  point. 
In  this  is  the  central  round  aperture  for  the  gas,  and  just 
ahead  of  it  are  four  lateral  apertures  for  the  air  supply. 
The  acetylene  and  air  mix  just  in  front  of  the  constric- 
tion and  the  two  burners  unite  their  jets  to  form  a  small, 
flat  flame.  It  is  in  effect  a  pair  of  tiny  Bunsen  burners 
inclined  to  produce  a  fishtail  jet. 

Larger  acetylene  burners  are  worked  on  a  similar  prin- 
ciple, all  having  the  air  supply  passages  characteristic  of 


8o 


THE   ART   OF   ILLUMINATION. 


the  Bunsen  burner.  Too  great  air  supply  for  the  acety- 
lene gives  the  ordinary  colorless  Bunsen  flame,  but  on  re- 
ducing the  amount  the  acetylene  burns  with  a  singularly 
white,  brilliant,  and  steady  flame. 

Of    acetylene    generators    designed    automatically    to 
supply  gas  at  constant  pressure  from  the  calcic  carbide  the 


Fig.  24. — Small  Acetylene  Generator. 

name  is  legion.  A  vast  majority  of  those  in  use  at  present 
are  of  rather  small  capacity,  being  designed  for  a  few 
lights  locally  or  as  portable  apparatus  for  lamps  used  for 
projection.  Generators  on  a  large  scale  have  hardly  come 


THE   MATERIALS    OF   ILLUMINATION.        81 

into  use,  and  the  problems  of  continuous  generation  have 
consequently  not  been  forced  into  prominence. 

A  very  useful  type  of  the  small  generator  is  shown  in 
Fig.  24,  a  form  devised  by  d'Arsonval.  It  consists  of  a 
small  gasometer  with  suitable  connections  for  taking  off 
the  gas  and  drawing  off  the  water.  The  bell  of  the 
gasometer  is  furnished  at  the  top  with  a  large  aperture 
closed  by  a  water  seal.  Through  this  is  introduced  a  deep 
iron  wire  basket  containing  the  charge  of  carbide. 

The  acetylene  is  generated  very  steadily  after  the  appa- 
ratus gets  to  working  and  the  pressure  is  quite  uniform. 
The  water  in  the  gasometer  of  the  d'Arsonval  machine  is 
covered  by  a  layer  of  oil,  which  serves  an  important  pur- 
pose. When  one  ceases  using  the  gas  the  bell  rises,  and  as 
the  carbide  basket  rises  out  of  the  water  the  oil  coats  it  and 
displaces  the  water,  checking  further  evolution  of  gas. 
The  oil  also  checks  evaporation,  so  that  there  is  no  slow 
evolution  of  gas  from  the  absorption  of  aqueous  vapor. 

As  to  the  value  of  acetylene,  it  is  evidently  worth 
about  fifteen  times  as  much  per  cubic  foot  as  gas  burned 
in  ordinary  burners,  or  three  to  four  times  as  much  as 
gas,  assuming  it  to  be  burned  in  Welsbach  burners. 
Now  one  ton  of  calcic  carbide  of  high  quality,  efficiently 
used,  will  produce  nearly  10,000  cu.  ft.  of  acetylene, 
equal  in  illuminating  v,alue  to  150,000  cu.  ft.  of  gas  in  the 
one  case  or  to  30,000  to  40,000  cu.  ft.  in  the  other. 

The  cost  of  the  calcic  carbide  is  a  very  uncertain  quan- 
tity at  present.  The  best  authorities  bring  the  manu- 
facturing cost,  on  a  large  scale  and  under  very  favorable 
circumstances,  somewhere  between  $30  and  $40  per  ton. 
It  is  doubtful  if  any  finds  its  way  into  the  hands  of  bona 
fide  users  at  less  than  about  $60  per  ton,  and  the  current 
price  in  small  lots  is  much  higher,  and  naturally  so,  by 


82  THE   ART   OF   ILLUMINATION. 

reason  of  troublesome  storage  and  the  cost  of  transporta- 
tion. Adding  the  necessary  allowance  for  the  cost  of 
producing  the  gas  from  the  carbide,  it  is  at  once  evident 
that  the  cost  of  lighting  by  acetylene  falls  below  that  of 
lighting  by  common  gas  in  ordinary  burners  at  the  com- 
mon price  of  $i  to  $1.50  per  1000  ft. 

It  is  equally  evident  that  it  considerably  exceeds  the  cost 
of  gas  lighting  by  Welsbach  burners.  There  seems  to  be 
small  chance  of  its  coming  into  general  competition  with 
either  at  present.  Its  cost  of  production  and  distribution 
does  not  yet  render  it  commercially  attractive  under  ordi- 
nary conditions. 

Nevertheless,  acetylene  is  for  use  in  isolated  places  one 
of  the  very  best  and  most  practical  illuminants,  for  it  is 
fairly  cheap,  easily  made,  and  gives  a  light  not  surpassed 
in  quality  by  any  known  artificial  illuminant.  It  is 
peculiarly  well  adapted  for  temporary  and  portable  use, 
giving  as  it  does  a  very  brilliant  and  steady  light,  well 
suited  for  use  with  reflectors  and  projecting  apparatus, 
admirable  in  color,  and  very  easy  of  operation. 


CHAPTER  V. 

THE     MATERIALS    OF     ILLUMINATION INCANDESCENT 

BURNERS. 

THE  general  class  of  illuminants  operative  by  the  in- 
candescence of  a  fixed  solid  body  would  include  in  prin- 
ciple both  arc  and  incandescent  electric  lamps,  as  well  as 
those  in  which  the  radiant  substance  is  heated  by  ordinary 
means.  In  this  particular  place,  however,  it  seems  appro- 
priate to  discuss  the  latter  forms  only,  leaving  the  electric 
lights  for  a  separate  chapter. 

Incandescent  radiants  brought  to  the  necessary  high 
temperature  by  a  non-luminous  flame  have  their  origin  in 
the  so-called  "  Drummond  "  or  "  lime  "  light,  which  has 
been  used  for  many  years  as  the  chief  illuminant  in  pro- 
jection, scenic  illumination  on  the  stage,  and  such  like  pur- 
poses, and  which  has  only  recently  been  extensively  re- 
placed by  the  electric  arc.  The  limelight  consists  of  a 
short  pencil  of  lime  against  which  is  directed  the  colorless 
and  intensely  hot  flame  from  a  blast  lamp  fed  with  pure 
oxygen  and  hydrogen,  or  more  commonly  with  oxygen 
and  illuminating  gas. 

The  general  arrangement  of  the  oxy-hydrogen  burner 
is  shown  in  Fig.  25.  Here  A  and  B  are  the  supply  pipes 
for  the  oxygen  and  hydrogen,  fitted  with  stop-cocks. 
These  unite  in  a  common  jet  in  the  burner  E,  which  is 
usually  inclined  so  as  to  bring  the  burner  where  it  will  not 
cast  a  shadow.  Sometimes  the  two  gases  are  mixed  in 
the  burner  tube  Cf  and  sometimes  the  hydrogen  is  deliv- 


84 


THE   ART   OF   ILLUMINATION. 


ered  through  an  annular  orifice  about  a  central  tube  which 
supplies  the  oxygen.  The  pencil  of  lime  is  carried  on  a 
holder  D,  and  the  whole  burner  is  often  carried  on  an  ad- 
justable stand  E,  so  that  it  can  be  raised,  lowered,  or 


Fig.  25. — Oxy-hydrogen  Burner. 

turned,  as  occasion  demands.  Themixea  gases  unite  in  a 
colorless,  slender  flame  of  enormously  high  temperature, 
and  when  this  impinges  on  the  lime  the  latter  rises  in  a 
small  circular  spot  to  the  most  brilliant  incandescence, 
giving  an  intense  white  light  of,  generally,  200  to  400 
candle-power. 

The  light,  however,  falls  off  in  brilliancy  quite  rapidly, 


THE  MATERIALS   OF   ILLUMINATION.        85 

particularly  when  the  initial  incandescence  is  very  intense, 
losing  something  like  two-thirds  of  its  candle-power  in  an 
hour,  so  that  it  is  the  custom  for  the  operator  to  turn  the 
pencil  from  time  to  time  so  as  to  expose  new  portions  to 
the  oxy-hydrogen  jet. 

At  the  highest  temperatures  the  calcium  oxide  is  some- 
what volatile  and  the  surface  seems  to  change  and  lose  its 
radiative  power.  Sometimes  pencils  of  zirconium  oxide 
are  used  instead  of  lime,  and  this  substance  has  proved 
more  permanently  brilliant  and  does  not  seem  to  volatilize. 
When  properly  manipulated,  the  calcium  light  is  beauti- 
fully steady  and  brilliant,  and  being  very  portable,  is  well 
adapted  for  temporary  use. 

From  time  to  time  attempts  were  made  to  produce  a 
generally  useful  incandescent  lamp  in  which  the  oxy- 
hydrogen  jet  should  be  replaced  by  a  Bunsen  burner  re- 
quiring only  illuminating  gas  and  air. 

Platinum  gauze  and  other  substances  were  tried  as  the 
incandescent  materials,  but  the  experiments  came  to  noth- 
ing practically  until  the  mantle  burner  of  Auer  von  Wels- 
bach  appeared.  This  is  generally  known  in  this  country 
as  the  Welsbach  light,  but  on  the  Continent  as  the  Auer 
light.  In  this  burner  the  material  brought  to  incandes- 
cence is  a  mantle,  formed  like  a  little  conical  bag,  of  thin 
fabric  thoroughly  impregnated  with  the  proper  chemicals 
and  then  ignited,  leaving  a  coarse  gauze  formed  of  the 
active  material. 

The  composition  of  this  material  has  been  kept  more  or 
less  secret,  and  has  been  varied  from  time  to  time  as  the 
burner  has  gradually  been  evolved  into  its  present  state, 
but  is  well  known  to  consist  essentially  of  the  oxides  of 
the  so-called  "metals  of  the  rare  earths,"  chiefly  thorium 
and  yttrium. 


86  THE   ART    OF   ILLUMINATION. 

These  rare  earths,  zirconia,  thoria,  glucina,  yttria,  and 
a  half-dozen  others  still  less  well  known,  form  a  very  curi- 
ous group  of  chemical  substances.  They  are  whitish  or 
yellowish  very  refractory  oxides  occurring  as  components 
of  certain  rare  minerals,  and  most  of  them  rise  to  magnifi- 
cent incandescence  when  highly  heated.  The  hue  of  this 
incandescence  differs  slightly  for  the  different  earths  and 
they  are  very  nearly  non-volatile  except  at  enormous 
temperatures.  One,  erbia,  has  the  extraordinary  property 
of  giving  a  spectrum  of  bright  bands  when  highly  heated 
instead  of  the  continuous  spectrum  usual  to  incandescent 
solids,  a  property  which  is  shared  in  less  degree  by  a  few 
of  its  curious  associates. 

The  mantle  burners  of  the  Welsbach  type  are  formed  of 
various  blends  of  the  more  accessible  of  these  rare  earths, 
and  when  brought  to  incandescence  by  the  flame  of  a  Bun- 
sen  burner  within  the  mantle,  give  a  most  brilliant  light 
with  a  very  small  expenditure  of  gas. 

As  first  manufactured  the  mantles  were  very  fragile, 
breaking  on  the  smallest  provocation,  but  they  have  gradu- 
ally been  increased  in  strength  until  those  now  made  gen- 
erally hold  together  for  many  hundred  hours,  and  usually 
should  be  discarded  for  inefficiency  long  before  they  break. 
This  statement  refers  to  mantles  burned  indoors  and  not 
subjected  to  any  unusual  vibration,  which  greatly  shortens 
their  life. 

As  at  present  manufactured  the  standard  Welsbach 
burner  complete  is  shown  in  Fig.  26,  of  which  the  several 
parts  are  distinctly  labeled  in  the  cut.  It  consists  es- 
sentially of  a  Bunsen  burner  with  provisions  for  regu- 
lating the  flow  of  both  air  and  gas,  capped  by  fine  wire 
gauze  to  prevent  the  flame  striking  back,  and  the  mantle 
within  which  the  Bunsen  flame  burns.  There  are  suita- 


THE   MATERIALS   OF   ILLUMINATION.        87 


ble  supports  for  the  chimney  and  shades  and  for  the 
mantle. 

The  mantle  carrier  is  permanently  attached  to  a  cap 
with  a  wire  gauze  top,  and  this  cap  goes  into  place  on 


—CHIMNEY 


SHADE  SUPPOn 


MANTLE 
MANTLESUPPORT 
CHIMNEY  SUPPORT 
GAUZE  TIP 
SOCKET 


SHADE '  SUPPORT 
ERY 


•BUNSENTUBE 
R SHUTTER 


GAS 

REGULATOR 


3UNSENTUBE 
Fig.  26. — Standard  Welsbach  Burner. 

the  burner  tube  with  a  bayonet  joint  so  that  the  mantle 
is  brought  exactly  to  the  right  place,  instead  of  having 
to  be  adjusted  over  a  permanent  cap.  This  is  one  of  the 
most  important  recent  improvements  in  this  type  of 
burner,  since  previously  the  risk  of  breakage  in  adjusting 
a  new  mantle  had  been  very  considerable. 

Several  makes  of  mantle  burners  are  in  use  at  the 
present  time,  but  the  ordinary  Welsbach  may  be  con- 


88 


THE   ART   O.F   ILLUMINATION. 


sidered  as  a  type  of  the  best  modern  practice,  and  the 
data  here  given  refer  to  it,  and  are  at  least  as  favorable 
as  would  be  derived  from  any  other  form. 

As  in  most  other  burners,  the  efficiency  of  the  mantle 
burner  increases  somewhat  with  the  capacity,  but  the 
general  result  reached  in  common  practice  with  16 
candle-power  (nominal)  gas  is  12  to  15  candle-power 
per  cubic  foot  of  gas,  assuming  the  mantle  to  be  new. 
In  other  words,  at  the  start  the  mantle  burner  is  nearly 


Fig.  27.— Life  Curves,  Welsbach  Mantles. 

five  times  as  efficient  as  an  Argand  burner,  about  six 
times  as  efficient  as  an  ordinary  burner,  and  two  to  three 
times  as  efficient  as  the  powerful  regenerative  burners. 
This  economy  is  not  maintained,  the  efficiency  of  the 
mantle  falling  off  with  use,  rapidly  at  first,  more  slowly 
afterwards.  This  is  due  in  part  to  actual  diminution  in 
the  radiating  surface  of  the  mantle  from  surface  disin- 
tegration and  in  part  to  real  decrease  in  the  radiant  ef- 
ficiency. Fig.  27  shows  a  set  of  life  curves  from  Wels- 
bach burners,  which  are  self-explanatory.  In  about  300 
hours  the  efficiency  has  fallen  off  nearly  one-third,  after 


THE   MATERIALS   OF  ILLUMINATION.        89 

which  it  decreases  much  less  rapidly  during  the  re- 
mainder of  the  life  of  the  mantle. 

This  decrease  in  efficiency  with  age  is  similar  to  that 
found  in  incandescent  electric  lamps,  but  is  initially  more 
rapid.  Nevertheless  even  after  300  hours  the  mantle  is 
still  good  for  8  or  10  candle-power  per  cubic  foot  of 
gas,  and  remains  far  more  efficient  than  any  other  class 
of  gas  burner.  Some  recent  mantles  are  even  more  ef- 
ficient than  these  figures  would  indicate. 

The  working  life  of  the  mantle  is  stated  by  Dr.  Fahn- 
drich,  director  of  gas  at  Vienna,  to  be  about  350  hours, 
taking  due  account  of  the  decrease  in  efficiency.  It  is 
safe  to  say  that  averaging  the  working  efficiency  over 
this  term  of  life  the  mantle  burner  with  gas  at  $i  per 
thousand  cubic  feet  can  be  operated  at  a  cost  not  ex- 
ceeding o.oi  cent  per  candle-hour  for  gas.  This  should 
not  be  raised  by  more  than  0.0025  cent  for  mantle  re- 
newals per  candle-hour.  The  upshot  of  the  matter  is 
that  the  mantle  burner  is  by  far  the  cheapest  known  il- 
luminant  except  the  electric  arc  at  a  rather  low  rate  for 
electrical  energy.  Obviously  it  uses  up  the  oxygen  and 
contaminates  the  air  only  in  proportion  to  the  gas  used, 
and  hence  far  less  than  other  burners. 

The  chief  objection  to  the  mantle  burner  is  the  un- 
pleasant greenish  tinge  of  its  light.  With  the  early 
burners  this  was  very  offensive,  and  even  with  the  latest 
forms  it  is  so  noticeable  that  one  can  walk  along  the 
street  and  pick  out  the  mantle  burners  by  the  greenish 
cast  of  the  illumination  long  before  reaching  the  window 
from  which  they  are  shining. 

The  exact  tinge  of  the  light  varies  a  little  with  the 
kind  of  mantle  and  the  particular  period  of  its  life,  but 
it  is  always  distinctly  greenish,  sometimes  bluish  green, 


9o  THE   ART   OF   ILLUMINATION. 

and  in  recent  mantles  sometimes  a  very  curious  shade  of 
yellowish  green,  but  never  yellowish  like  a  gas  flame  or 
an  incandescent  lamp,  or  white  or  bluish  white  like  an 
electric  arc. 

This  color  seems  thus  far  to  be  inseparable  from  the 
radiation  derived  from  any  feasible  combination  of  the 
rare  earths  used  to  form  the  mantle.  Sometimes  in  the 
youth  of  the  mantle  the  light  seems  to  be  nearly  free 
from  this  tinge,  but  through  change  in  the  specific  nature 
of  the  radiation  or  dissipation  of  some  of  the  components 
the  greenish  light  soon  gains  prominence.  Whether  this 
difficulty  can  be  overcome  in  the  manufacture  of  the  man- 
tles it  is  impossible  to  predict,  but  it  can  to  a  certain  ex- 
tent be  avoided  by  proper  shading,  and  shading  is  nearly 
always  necessary  in  using  mantle  burners  on  account  of 
their  great  intrinsic  brilliancy. 

If  the  exploiters  of  these  mantle  burners  had  spent  half 
the  time  in  devising  remedial  measures  that  they  have 
wasted  in  denying  the  greenish  hue  of  the  light  or  in  ex- 
plaining that  it  is  quite  artistic  and  really  good  for  the 
eyes,  the  ordinary  gas  burner  would  now  be  practically 
driven  out  of  use. 

As  regards  the  actual  color  of  the  light  from  mantle 
burners,  it  varies  somewhat,  as  already  explained,  but  the 
following  table  is  typical  of  the  peculiarities  of  the  light 
as  compared  with  that  from  an  ordinary  gas  flame.  In 
the  table  the  light  of  the  gas  flame  is  supposed  to  be 
unity  for  each  of  the  colors  concerned,  when  the  light 
from  the  mantle  has  the  given  relative  values. 

As  the  actual  luminosity  of  the  deep  reel,  blue,  and 
violet  is  comparatively  small  in  either  burner,  the  pre- 
ponderance of  green  in  the  light  from  the  mantle  is  very 
marked. 


THE  MATERIALS   OF   ILLUMINATION.        91 


Color   

FULL 

YELLOW 

YELLOWISH 

BLUISH 

BLUF 

VIOLET 

RED 

GREEN 

GREEN 

Light  from  Mantle  

.71 

I  47 

I   76 

2   TQ 

2  74 

o  no 

Ar^and  taken  as 

I  OO 

I  OO 

I  OO 

I  OO 

><->y 

To  correct  this  it  is  necessary  to  use  a  shade  of 
such  color  as  to  absorb  some  of  the  green  rays.  The 
actual  percentage  of  light  absorbed  need  not  be  at  all 
large,  provided  the  absorption  is  properly  selective. 
The  general  color  of  the  shade  to  effect  this  absorption 
will  generally  be  a  light  rose  pink,  and  the  result  is  a 
fairly  white  light,  better  in  color  than  an  ordinary  gas 
flame. 

The  advantage  of  the  mantle  burner  in  steadiness  and 
economy  is  so  great  that  there  would  be  little  reason  for 
using  the  more  common  forms  of  gas  burner  indoors,  ex- 
cept for  their  better  artistic  effects  and  for  their  con- 
venience for  very  small  lights.  The  color  question  and 
the  fragility  of  the  mantle  have  been  the  chief  hindrances 
to  the  general  introduction  of  the  Welsbach  type,  and 
these  are  certainly  in  large  measure  avertable. 

Recently  there  have  been  introduced  several  forms  of 
mantle  burner  worked  with  gas  generated  on  the  spot 
from  gasoline  or  similar  petroleum  products.  Some- 
times these  are  operated  as  individual  lamps  and  some- 
times as  small  systems  to  which  the  gas-forming  fluid 
is  piped.  They  give,  of  course,  a  fine,  brilliant  light,  and 
at  a  low  cost — cheaper  than  ordinary  mantle  burners 
worked  with  any  except  rather  cheap  gas.  Where  gaso- 
line gas  would  be  cheaper  than  gas  taken  from  the  near- 
est available  main,  such  gasoline  mantle  burners  will 
prove  economical. 

But,  as  a  matter  of  fact,  lamps  locally  generating  and 


92  THE   ART   OF   ILLUMINATION. 

burning  their  own  petroleum  gas  have  been  pretty  thor- 
oughly tried  from  time  to  time  during  the  past  twenty- 
five  years,  and  have  never  taken  a  strong  or  permanent 
hold  on  the  public.  It  is  therefore  difficult  to  see  how 
mantle  burners  worked  in  similar  fashion  are  likely  to  take 
a  material  hold  upon  the  art,  although  in  special  cases  they 
may  prove  very  useful,  when  illuminating  gas  is  not 
available  at  a  reasonable  price. 

It  must  be  constantly  borne  in  mind  that  the  lighter 
petroleum  oils  are  dangerous  and  must  be  used  with 
extreme  care,  and  also  that  they  are  just  now  rapidly 
rising  in  price,  owing  to  the  increasing  use  of  explosion 
engines  and  gas  machines. 

In  using  any  mantle  burner  it  is  good  economy  to 
replace  the  mantle  after  three  or  four  hundred  hours  of 
burning,  if  it  is  in  regular  use  to  any  considerable  extent. 
Of  course,  in  cases  when  a  burner  is  not  regularly  used 
and  its  maximum  brilliancy  is  not  at  all  needed  the  man- 
tle may  properly  be  used  until  it  shows  signs  of  break- 
ing. In  other  words,  as  soon  as  a  mantle  which  is  needed 
at  its  full  efficiency  gets  dim,  throw  it  promptly  away; 
but  so  long  as  it  gives  plenty  of  light  for  its  situation, 
your  consumption  of  gas  will  not  be  diminished  by  a 
change. 

The  commonest  trouble  with  mantles  is  blackening 
from  a  deposit  of  soot  owing  to  temporary  derangement 
of  the  burner.  This  deposit  can  generally  be  burned  off 
by  slightly,  not  considerably,  checking  the  air  supply 
so  as  to  send  up  a  long,  colorless  flame  which  will  soon 
get  rid  of  the  carbon,  after  which  the  full  air  supply 
should  be  restored.  Too  great  checking  of  the  air  sup- 
ply produces  a  smoky  flame. 

It  should  finally  be  noted  that  the  mantle  burners  are 


THE   MATERIALS   OF  ILLUMINATION. 


93 


particularly  useful  in  cases  of  troublesome  fluctuations 
in  the  gas  supply,  since  while  they  may  burn  more  or 
less  brightly  according  to  circumstances,  they  are  en- 
tirely free  from  flickering  when  properly  adjusted. 

In  leaving  now  the  illuminants  which  depend  upon 
the  combustion  of  a  gas  of  liquid,  a  brief  summation  of 
some  of  their  properties  may  not  come  amiss. 

The  replacement  of  candles  and  lamps  by  gas  worked 
a  revolution,  not  only  in  the  convenience  of  artificial 
lighting,  but  in  its  hygienic  relations.  The  older  illumi- 
nants in  proportion  to  their  luminous  effect  removed  pro- 
digious amounts  of  oxygen  from  the  air  and  gave  off 
large  quantities  of  carbonic  acid.  In  the  days  of  candles 
a  brilliantly  lighted  room  was  almost  of  necessity  one 
in  which  the  air  was  bad.  The  following  table,  due  to 
a  well-known  authority  on  hygiene,  gives  the  approxi- 
mate properties  of  the  common  illuminants  of  com- 
bustion as  regards  their  effects  on  the  air  of  the  space 
in  which  they  are  burned. 


Q 

Q 

O 

3 

U 

rt 

H 

s 

Ctf 

Q 

u 

a 

E> 

5 

U 

D 

y 

^,  5^ 

en  & 

1 

Q 

U 

Q 
O    . 

D  C/3 

D  W 

0-° 

OQ 

p 

^.  H 

Q  H 

3i  H 

Ld     T 

ua 

ft< 

^    • 

O  ^ 

ft.  fc. 

05  O 

ss'1' 

^  0$ 

jj 

sg 

OH   S 

S  "^ 

^  J 

OH 

Ha! 

Q 

go 

O 

0 

gU 

U 

ig 

g 

u 

O 

O 

X 

«  ** 

& 

- 

> 

Tallow  candles  

2200  grains 

16 

10.7 

7-3 

8.2 

1400 

12.  0 

Sperm  candles  

1740      " 

16 

9.6 

6-5 

1137 

II. 

Paraffin  oil         .          .... 

QQ2        " 

16 

6.2 

4.5 

3-5 

1030 

7  5 

Kerosene  oil  

w 

QOO         " 

16 

5.9 

4.1 

3-3 

1030 

7.0 

Coal  gas,  batwing  

VW7 

5.5  cu.ft. 

16 

6.5 

2.8 

7-3 

1194 

5.0 

Coal  gas    Argand                        . 

48    " 

16 

5.8 

2.6 

6.4 

1240 

4-3 

Coal  gas    Regenerative  

*r*  w 

3.2      " 

32 

3.6 

1.7 

760 

2.8 

Coal  gas,  Welsbach  

&m. 

3.5 

J 

50 

4.1 

1.8 

4.7 

763 

3.0 

3 

94  THE   ART   OF   ILLUMINATION. 

To  this  it  may  be  added  that  acetylene  in  these  rela- 
tions is- about  on  a  parity  with  the  Welsbach  burner,  and 
that  oil  lamps  other  than  kerosene,  burning  whale  oil, 
colza  oil,  etc.,  would  fall  in  just  after  candles.  It  is 
somewhat  startling  to  realize,  but  very  desirable  to  re- 
member, that  a  common  gas  burner  will  vitiate  the  air 
of  a  room  as  much  as  four  or  five  persons,  in  so  far,  at 
least,  as  vitiation  can  be  defined  by  change  in  the  chemical 
composition  of  the  air. 

In  cost  also  the  modern  illuminants  have  a  material 
advantage.  In  order  of  cost  the  list  would  run  at  cur- 
rent American  prices  of  materials  about  as  follows :  Can- 
dles, animal  and  vegetable  oils,  gas  in  ordinary  burners, 
kerosene,  acetylene,  Welsbachs.  Incandescent  electric 
lamps,  it  may  be  added,  are  about  equivalent  in  cost  to 
ordinary  gas,  with  a  tremendous  hygienic  advantage  in 
their  favor,  while  arc  lamps  would  be  the  lowest  on  the 
list,  assuming  electrical  energy  relatively  as  cheap  as  dol- 
lar gas  would  be.  As  to  the  quality  of  the  illumination, 
incandescent  lamps,  regenerative  gas  burners,  and  acety- 
lene lead  the  list,  while  Welsbachs,  by  reason  of  their 
color,  and  arc  lamps,  from  their  lack  of  steadiness,  would 
take  a  low  rank. 


CHAPTER   VI. 

THE    ELECTRIC     INCANDESCENT  LAMP. 

AT  the  present  time  the  mainstay  of  electric  illumina- 
tion is  the  incandescent  lamp,  in  which  a  filament  of 
high  electrical  resistance  is  brought  to  vivid  incandes- 
cence by  the  passage  of  the  electric  current.  To  prevent 
the  rapid  oxidation  of  the  filament  at  the  high  tem- 
perature employed,  the  filament  is  mounted  in  an  ex- 
hausted glass  globe,  forming  the  familiar  incandescent 
lamp  of  commerce. 

The  first  attempts  at  incandescent  lamps  were  made 
with  loops  or  spirals  of  platinum  wire  heated  by  the 
electric  current,  either  in  the  air  or  in  vacuo,  but  the 
results  were  highly  unsatisfactory,  since  in  the  open  air 
the  wire  soon  began  to  disintegrate,  and  even  in  the 
absence  of  air  its  life  was  short.  Moreover,  the  metal 
itself,  being  produced  in  very  limited  quantities,  was 
expensive  at  best,  and  rose  very  rapidly  in  price  under 
a  small  increase  of  demand.  Having  a  fairly  low  specific 
electrical  resistance,  the  wire  used  had  either  to  be  very 
thin,  which  made  it  extremely  fragile,  or  long,  which 
greatly  increased  its  cost. 

Following  platinum  came  carbon  in  the  form  of  slen- 
der pencils  mounted  in  vacuo.  These,  however,  were  of 
so  low  resistance  that  the  current  required  to  heat  them 
was  too  great  to  allow  of  convenient  distribution. 

To  get  a  practical  lamp  it  was  necessary  to  use  a  fila- 

95 


96  THE   ART   OF   ILLUMINATION. 

ment  of  really  high  resistance,  and  which  was  yet  strong 
enough  to  keep  down  the  cost  of  replacements. 

Without  going  into  the  details  of  the  many  experi- 
ments on  incandescent  lamps,  it  is  sufficient  to  say  that 
after  much  labor  the  problem  of  getting  a  fairly  worka- 
ble filament  was  solved  through  the  persistent  efforts  of 
Edison,  Swan,  Maxim,  Weston,  and  others,  about  twenty 
years  ago,  the  modern  art  dating  from  about  1880. 

All  the  recent  filaments  are  based  on  the  carbonization, 
out  of  contact  with  air,  of  thin  threads  of  cellulose — 
the  essential  constituent  of  woody  fiber.  The  early  work 
was  in  the  direction  of  carbonizing  thread  in  some  form, 
or  even  paper,  but  Edison,  after  an  enormous  amount  of 
experimenting,  settled  upon  bamboo  fiber  as  the  most 
uniform  and  enduring  material,  and  the  Edison  lamp 
came  to  the  front  commercially. 

In  point  of  fact,  it  soon  became  evident  that  art  could 
produce  a  far  more  uniform  carbon  filament  than  nature 
has  provided,  so  that  of  late  years  bamboo,  thread, 
paper,  and  the  rest  have  been  abandoned,  and  all  fila- 
ments, save  those  for  some  special  lamps  of  large  candle- 
power,  are  made  from  soluble  cellulose  squirted  into 
threads,  hardened,  carbonized,  and  "  treated." 

Fig.  28  shows  a  typical  modern  incandescent  lamp. 
It  consists  essentially  of  four  parts;  the  base  adapted 
to  carry  the  lamp  in  its  socket,  the  bulb,  the  filament, 
and  the  filament  mounting,  which  includes  the  lead- 
ing-in  wires.  In  its  original  form  the  bulb  has  an  open- 
ing at  each  end,  one  at  the  base  end  through  which  the 
filament  and  its  mounting  are  put  in  place,  and  another 
in  the  form  of  a  narrow  tube  a  few  inches  long,  which 
when  sealed  off  produces  the  tip  at  the  end  of  the  bulb. 

The  filament  is  made  in  slightly  different  ways  in  dif- 


THE  ELECTRIC  INCANDESCENT  LAMP.      97 

ferent  factories,  and  the  exact  details  of  the  process, 
constantly  subject  to  slight  improvements,  are  unneces- 


Fig.  28. — Typical  Incandescent  Lamp. 

sary  here  to  be  described.    Substantially  it  is  as  follows: 
The  basis  of  operations   is   the   purest  cellulose   con- 


98  THE   ART   OF   ILLUMINATION. 

venient  to  obtain,  filter  paper  and  the  finest  absorbent 
cotton  being  common  starting  points.  The  material  is 
pulped,  as  in  paper  making,  dissolved  in  some  suitable 
substance,  zinc  chloride  solution  being  one  of  those  used, 
evaporated  to  about  the  consistency  of  thick  molasses, 
and  then  squirted  under  air  pressure  into  a  fine  thread, 
which  is  received  in  an  alcohol  bath  to  harden  it. 

Thus  squirted  through  a  die  the  filament  is  of  very 
uniform  constitution  and  size,  and  after  carbonization 
out  of  contact  with  air  it  forms  a  carbon  thread  that  is 
wonderfully  flexible  and  strong.  But  even  so,  there  is 
not  yet  a  perfectly  uniform  filament,  and  the  carbon  is 
not  dense  and  homogeneous  enough  to  stand  protracted 
incandescence. 

On  passage  of  current  portions  of  the  filament  may 
show  too  low  resistance,  so  as  to  be  dull,  or  too  high 
resistance,  so  as  to  get  too  hot  and  burn  off.  It  is  hard, 
too,  to  produce  a  durable  filament  of  the  somewhat 
porous  carbon  obtained  in  the  way  described. 

In  making  up  the  filaments  they  are  therefore  sub- 
jected prior  to  being  sealed  into  the  lamp  to  what  is 
known  as  the  flashing  process.  This  has  a  twofold  ob- 
ject, to  build  up  the  filament  with  dense  carbon,  and  to 
correct  any  lack  of  uniformity  which  may  exist.  The 
latter  purpose  is  far  less  important  to  the  squirted  fila- 
ments than  to  the  old  filaments  of  bamboo  fiber  or 
thread,  but  the  former  is  important  in  securing  a  uni- 
form product.  The  filaments  are  mounted  and  then  are 
gradually  brought  to  vivid  incandescence  in  an  atmos- 
phere of  hydrocarbon  vapor,  produced  from  gasoline  or 
the  like. 

The  heated  surface  decomposes  the  vapor,  and  the 
carbon  is  deposited  upon  the  filament  in  the  form  of  a 


THE  ELECTRIC   INCANDESCENT  LAMP.      99 

smooth  uniform  coating  almost  as  dense  as  graphite, 
and  a  considerably  better  conductor  than  the  original 
filament.  If,  as  in  the  early  bamboo  filaments,  there  are 
any  spots  of  poorer  conductivity  or  smaller  cross  section 
than  is  proper,  these  become  hot  first  and  are  built  up 
toward  uniformity  as  the  current  is  gradually  raised, 
so  that  the  filament  is  automatically  made  uniform. 

The  flashing  process  is  actually  quick,  the  gradual  rise 
of  current  being  really  measured  by  seconds.  With  the 
squirted  filaments  now  used  the  main  value  of  the  flash- 
ing process  is  to  enable  the  conductivity  of  the  filament 
to  be  quite  accurately  regulated,  at  the  same  time  giving 
it  a  firm,  hard  coating  of  carbon  that  greatly  increases 
its  durability.  The  finished  filaments  are  strong  and 
elastic,  generally  a  fine  steely-gray  in  color,  with  a  pol- 
ished surface,  and  for  lamps  of  ordinary  candle-power 
and  voltage  vary  from  6  to  12  ins.  in  length,  with  a 
diameter  of  5  to  10  one-thousandths  of  an  inch. 

The  filaments  are  joined  near  the  base  of  the  lamp  to 
two  short  bits  of  thin  platinum  wire  which  are  sealed 
through  one  end  of  a  short  piece  of  glass  tube.  Some- 
times these  platinum  leading-in  wires  are  fastened  di- 
rectly to  the  ends  of  the  filament  and  sometimes  to  an 
intermediary  terminal  of  copper  wire  attached  to  the  fila- 
ment. Within  the  tube  the  platinum  wires  are  welded 
to  the  copper  leads  which  pass  down  the  mounting  tube 
and  are  attached  to  the  base.  The  filament  itself  is 
cemented  to  its  copper  or  platinum  wires  by  means  of  a 
little  drop  of  carbon  paste. 

No  effective  substitute  for  platinum  in  sealing  through 
the  glass  has  yet  been  found,  although  many  have  been 
tried.  Platinum  and  glass  have  very  nearly  the  same 
coefficient  of  expansion  with  heat,  so  that  the  seal  re- 


ioo  THE   ART    OF   ILLUMINATION. 

mains  tight  at  all  temperatures  without  breaking  away. 
It  is  possible  to  find  alloys  with  nearly  the  right  coefficient 
of  expansion,  but  they  have  generally  proved  unsatis- 
factory either  mechanically  or  electrically,  so  that  the 
line  of  improvement  has  mainly  been  in  the  direction  of 
making  a  very  short  seal  with  platinum  wires. 

The  filament  thus  mounted  is  secured  in  the  bulb  by 
sealing  the  base  of  the  mounting  tube  or  lamp  stem  into 
the  base  of  the  bulb.  This  leaves  the  bulb  closed  except 
for  the  exhaustion  tube  at  its  tip. 

The  next  step  is  the  exhaustion  of  the  bulb.  This 
used  to  be  done  almost  entirely  by  mercury  pumps,  and 
great  pains  was  taken  to  secure  a  very  high  degree  of 
exhaustion.  It  was  soon  found  that  there  was  such  a 
thing  as  too  high  exhaustion,  but  the  degree  found  to 
be  commercially  desirable  is  still  beyond  the  easy  capa- 
bilities of  mechanical  air  pumps,  at  least  for  regular  and 
uniform  commercial  practice,  although  they  have  been 
sometimes  successfully  used. 

At  the  present  time  the  slow  though  effective  mer- 
cury pump  is  being  to  a  very  large  extent  superseded 
by  the  Malignani  process,  or  modifications  thereof.  The 
bulbs  are  rapidly  exhausted  by  mechanical  air  pumps, 
and  when  these  have  reached  the  convenient  limit  of 
their  action  the  residual  oxygen  is  chemically  absorbed 
by  the  gas  produced  by  the  vaporization  of  a  small  quan- 
tity of  a  solution  previously  placed  in  a  tubulaire  con- 
nected with  the  exhaustion  tube.  The  exact  nature  of 
the  solution  used  is  at  the  present  time  a  trade  secret, 
but  phosphorus  and  iodine  are  said  to  form  the  basis 
of  its  composition.  The  process  is  cheap,  rapid,  and 
effective,  and  with  a  little  practice  the  operator  can 
produce  exhaustion  that  is  almost  absolutely  uniform. 


THE  ELECTRIC   INCANDESCENT   LAMP.     101 

Whatever  be  the  method  of  exhaustion,  during  its 
later  stages  current  is  put  on  the  filaments  both  to  heat 
them,  and  thus  to  drive  out  the  occluded  gases,  and  to 
serve  as  an  index  of  the  exhaustion.  When  exhaustion 
is  complete  the  leading-in  tube  is  quickly  sealed  off,  and 
the  lamp  is  done,  save  for  cementing  on  the  base  and 
attaching  it  to  the  leads  that  come  from  the  seal.  After 
this  the  lamps  are  sorted,  tested,  and  made  ready  for 
the  market. 

The  shape  of  the  filament  in  the  lamp  was  originally 
a  simple  U,  later  often  modified  to  a  U  with  a  quarter- 
twist  so  that  the  plane  of  the  loop  at  the  top  was  90  de- 
grees from  its  plane  at  the  base.  As  the  voltage  of 
distribution  has  steadily  crept  upwards  from  100  to  no, 
1 20,  140,  and  even  250  volts,  it  has  been  necessary  either 
to  increase  the  specific  resistance  of  the  filament,  to  de- 
crease its  diameter,  or  to  increase  its  length,  in  order  to 
get  the  necessary  resistance  to  keep  the  total  energy, 
and  likewise  the  temperature  of  the  filament,  down  to 
the  desired  point. 

But  the  modern  flashed  filament  cannot  be  greatly  in- 
creased in  specific  resistance  without  impairing  its  sta- 
bility, so  the  filaments  have  been  growing  steadily  finer 
and  longer.  At  present  their  form  is  various,  accord- 
ing to  the  judgment  of  the  maker  in  stowing  away-  the 
necessary  amount  of  filament  within  the  bulb. 

One  very  common  form  is  that  of  Fig.  28,  where  the 
filament  has  a  single  long  convolution  anchored  to  the 
base  at  its  middle  point  for  mechanical  steadiness. 
Sometimes  there  are  two  convolutions,  or  even  more, 
and  sometimes  there  is  merely  a  reduplication  of  the  old- 
fashioned  simple  loop,  as  in  Fig.  29. 

The  section  of  the  filaments  is  now  always  circular, 


102  THE   ART   OF   ILLUMINATION. 

although  in  the  early  lamps  they  were  sometimes  rec- 
tangular or  square. 

There  has  been  a  considerable  fog  of  mystery  about 
incandescent  lamp  practice  for  commercial  purposes,  but 


Fig.  29. — Lamp  with  Double  Filament. 

the  general  facts  are  very  firmly  established  and  by  no 
means  complicated,  and  a  little  consideration  of  them 
will  clear  up  much  of  the  haze. 

To  begin  with,  it  is  not  difficult  to  make  a  good  fila- 


THE  ELECTRIC   INCANDESCENT   LAMP.     103 

ment,  but  it  takes  much  skill  and  practice  to  produce,  in 
quantity,  one  that  shall  be  uniformly  good.  The  quality 
of  the  lamps  as  to  durability  and  other  essentials  de- 
pends very  largely  on  the  care  and  conscientiousness  of 
the  maker  in  sorting  and  rating  his  product. 

It  is  practically  impossible,  for  example,  to  make,  say, 
10,000  filaments,  all  of  which  shall  give  15  to  17  hori- 
zontal candle-power  at  a  particular  voltage,  say,  no. 
With  great  skill  in  manufacture,  half  or  rather  more  will 
fall  within  these  limits,  the  rest  requiring  anywhere  be- 
tween 100  and  120  volts  to  give  that  candle-power. 
Only  a  few  will  reach  these  extremes,  the  rest  being 
clustered  more  or  less  closely  around  the  central  point. 

The  value  of  the  lamps  as  sold  depends  largely  on 
what  is  done  with  the  varying  ones  and  how  carefully 
they  are  sorted  and  rated.  If  the  lamps  demanded  on 
the  market  were  all  of  no  volts,  then  there  would  be 
a  large  by-product  which  would  either  have  to  be  thrown 
away,  sold  for  odd  lamps  of  uncertain  properties,  or 
slipped  surreptitiously  into  lots  of  standard  lamps. 

But  some  companies  use  lamps  of  1 08  or  1 12,  or  some 
neighboring  voltage,  and  part  of  the  product  is  exactly 
fitted  to  their  needs,  and  so  forth,  there  being  involved 
only  some  slight  difference  in  efficiency,  not  important  if 
similar  lamps  from  other  lots  are  conscientiously  rated 
along  with  them. 

The  basic  facts  in  incandescent  lamp  practice  are  two: 
First,  the  efficiency,  i.  c.,  the  ratio  of  energy  consumed 
to  light  given  per  unit  of  surface,  depends  mainly  on  the 
temperature  to  which  the  filament  is  carried;  second,  the 
total  light  given  is  directly  proportional  to  the  filament 
surface  which  radiates  this  light.  The  specific  radiating 
power  of  modern  carbon  filaments  is  substantially  the 


io4  THE   ART    OF   ILLUMINATION. 

same,  so  that  if  one  has  two  filaments  of  the  same  sur- 
face brought  to  the  same  temperature  of  incandescence 
they  will  work  at  substantially  the  same  efficiency  and 
give  substantially  the  same  amount  of  light. 

And  if  a  filament  of  a  certain  surface  be  brought  to  a 
certain  temperature  it  will  give  a  definite  total  amount  of 
light,  utterly  irrespective  of  the  form  in  which  the  fila- 
ment is  disposed.  Changes  in  the  form  of  the  filament 
will  produce  changes  in  the  distribution  of  the  light  in 
different  directions  around  the  lamp,  but  will  not  in  the 
least  change  the  total  luminous  radiation.  Much  of  the 
current  misunderstanding  is  due  to  neglect  of  this  sim- 
ple fact. 

The  nominal  candle-power  of  the  lamp  depends  upon 
a  pure  convention  as  to  the  direction  and  manner  in 
which  the  light  shall  be  measured  in  rating  the  lamp, 
and  makers  have  often  sought  to  beat  the  game  by  dis- 
posing the  filament  so  as  to  exaggerate  the  radiation  in 
the  conventional  direction  of  measurement. 

For  example:  Many  early  incandescent  lamps  had 
filaments  of  square  cross  section  bent  into  a  single  sim- 
ple U.  These  gave  their  rated  candle-power  in  direc- 
tions horizontally  45  degrees  from  the  plane  of  the  fila- 
ments, and  this  was  the  maximum  in  any  direction,  so 
that  the  lamp  when  thus  measured  was  really  credited 
with  its  maximum  candle-power,  and  fell  below  its  rat- 
ing in  all  directions  save  the  four  horizontal  directions 
just  noted. 

It  is  customary  to  delineate  the  light  from  an  incan- 
descent lamp  in  the  form  of  closed  curves,  of  which  the 
various  radii  represent  in  direction  and  length  the  rela- 
tive candle-power  in  those  various  directions.  Such 
curves  may  be  made  to  show  accurately  the  distribution 


THE  ELECTRIC  INCANDESCENT  LAMP.     105 

of  light  in  a  horizontal  plane  about  the  lamp,  or  the 
distribution  in  any  vertical  plane,  and  from  the  average 
radii  in  any  plane  may  be  deduced  the  mean  candle- 
power  in  that  plane,  while  from  a  combination  of  the 


Fig.  30.— Distribution  of  Light  from  Flat  Filament. 

radii  in  the  various  planes  may  be  obtained  the  mean 
spherical  candle-power  which  measures  the  total  lumi- 
nous radiation  in  all  directions. 

This  last  is  the  true  measure  of  the  total  light-giving 
power  of  a  lamp.  Fig.  30  illustrates  the  curve  of  hori- 
zontal distribution  for  one  of  the  early  lamps,  having  a 
flat  U-shaped  filament.  The  circle  is  drawn  to  show  a 
uniform  16  candle-power,  while  the  irregular  curve 
shows  the  actual  horizontal  distribution  of  light.  This 
particular  lamp  overran  its  rating,  but  its  main  char- 
acteristic is  that  it  gave  a  strong  light  in  one  horizontal 
diameter  and  a  weak  one  in  the  diameter  at  right  angles 
to  this. 


io6 


THE   ART   OF   ILLUMINATION. 


Such  a  distribution  as  this  is  generally  objectionable, 
and  most  modern  filaments  are  twisted  or  looped,  so 
that  the  horizontal  distribution  is  nearly  circular. 
Fig.  31  shows  a  similar  curve  for  a  recent  i6-cp  lamp 
of  the  type  shown  in  Fig.  28.  In  the  small  inner  circle 
is  shown  the  projection  of  the  looped  filament  as  one 
looks  down  upon  the  top  of  the  lamp.  Fig.  32  shows 


Horizontal  Distribution 


Vertical  on SD... Horizontal 


Figs.  31  and  32. — Distribution  of  Light  from  Looped  Filament. 

a  similar  delineation  of  the  distribution  of  light  in  a 
vertical  plane  taken  in  the  azimuth  shown  in  Fig.  31, 
with  the  socket  up. 

The  looping  of  the  filament  is  such  that  the  horizontal 
distribution  is  very  uniform,  while  in  the  vertical  down- 
wards there  is  a  marked  diminution  of  light,  and  of 
course  in  the  direction  of  the  socket  much  of  the  light  is 
cut  off.  The  total  spherical  distribution,  if  one  can  con- 
ceive it  laid  out  in  space  in  three  dimensions,  resembles 
a  very  flat  apple  with  a  marked  depression  at  the 
blossom  end  and  a  cusp  clear  in  to  the  center  at  the 
stem  end.  Fig.  33  is  an  attempt  to  display  this  spherical 
distribution  to  the  eye. 

If  the  filament  were  a  simple  U  or  the  double  U  of  Fig. 


THE   ELECTRIC   INCANDESCENT  LAMP.     107 

29,  assuming  the  same  total  length  and  temperature  of 
filament,  the  apple  would  have  still  greater  diameter,  but 
the  depression  at  the  blossom  end  would  be  considerably 
wider  and  deeper. 

If  the  filament  has  several  convolutions,  as  in  Fig.  34, 
this  depression  is  considerably  reduced,  but  there  is  a 


Fig.  33. — Distribution  of  Light  from  Incandescent  Lamp. 

marked  flattening  in  one  horizontal  direction,  so  that 
the  horizontal  distribution  would  somewhat  resemble 
Fig.  30.  But  the  total  luminous  radiation  would  be 
quite  unchanged. 

If  the  lamps  were  rated  by  their  mean  horizontal 
candle-power  the  U  filament  would  show  abnormally 
large  horizontal  illumination  for  the  energy  consumed, 
and  would  apparently  be  very  efficient,  while  if  one  were 
foolish  enough  to  rate  lamps  by  the  light  given  off  the 


io8 


THE   ART   OF  'ILLUMINATION. 


tip  alone,  Fig.  34  would  show  great  efficiency,  the  distri- 
bution in  one  horizontal  diameter  having  been  reduced 
to  fatten  the  curve  at  the  tip.  In  reality,  however,  each 
one  of  the  three  forms  of  lamp  would  have  exactly  the 


Fig.  34. — Lamp  with  Multiple-Looped  Filament. 

same  efficiency,  and  in  practice  there  would  be  little 
choice  between  them. 

In  the  every-day  work  of  illumination  incandescent 
lamps  arc  installed  with  their  axes  in  every  possible  di- 
rection, the  vertical  being  the  rarest,  and  angles  between 
30  degrees  and  60  degrees  downwards  from  the  hori- 
zontal the  commonest. 

Bearing  in  mind  this  general  distribution  of  the  axes 
and  the  fact  that  diffusion  goes  very  far  toward  oblit- 


THE  ELECTRIC   INCANDESCENT   LAMP.     109 

crating  differences  in  the  spherical  distribution  as  re- 
gards general  illumination,  it  is  easy  to  see  that  the  shape 
of  the  filament  is,  for  practical  purposes  of  illumination, 
of  little  account.  In  the  few  cases  where  directed  il- 
lumination is  needed  it  is  best  secured  by  a  proper  re- 
flector, which  gives  far  better  results  than  can  be  ob- 
tained by  juggling  with  the  shape  of  the  filament. 

The  thing  of  importance  is  to  get  uniform  filaments 
of  first-class  durability*  and  of  as  good  efficiency  as  pos- 
sible. The  only  proper  test  for  efficiency,  however,  is 
that  based  on  mean  spherical  candle-power,  since  a 
lamp  will  give  a  different  apparent  efficiency  for  each 
direction  of  measurement,  varying  from  zero  in  the  di- 
rection of  the  socket  to  a  maximum  in  some  direction 
unknown  until  found. 

Efficiency  has  most  often  been  taken  with  respect  to 
the  mean  horizontal  candle-power.  But  this  leads  to 
correct  relative  results  only  when  comparing  lamps  hav- 
ing filaments  similarly  curved.  The  mean  spherical 
candle-power  is  usually  from  80  to  85  per  cent,  of  the 
mean  horizontal  candle-power,  a  ratio  larger  than  is 
found  in  the  case  of  any  other  artificial  illuminant. 

As  regards  efficiency,  most  commercial  incandescent 
lamps  require  between  3  and  4  watts  per  mean  horizontal 
candle-power.  Now  and  then  lamps  are  worked  at  2.5 
watts  per  candle  when  used  with  storage  batteries,  and 
some  special  lamps,  especially  some  of  those  made  for 
voltages  above  200,  range  over  4  watts  per  candle.  As 
has  already  been  remarked,  the  efficiency  depends  upon 
the  temperature  at  which  the  carbon  filament  is  worked. 
And  it  is  in  the  ability  to  stand  protracted  high  tem- 
perature that  filaments  vary  most. 

It  is  comparatively  easy  to  make  a  filament  which  will 


no  THE   ART   OF   ILLUMINATION. 

stand  up  well  when  worked  at  4  watts  per  candle,  but 
to  make  a  good  3-watt  per  candle  filament  is  a  very 
different  proposition.  Also,  at  low  voltage,  50  volts 
for  instance,  the  filament  is  more  substantial  than  the 
far  slenderer  one  necessary  to  give  the  requisite  resist- 
ance for  use  at  the  same  candle-power  at  100  or  125 
volts. 

Under  protracted  use  the  filament  loses  substance  by 
slow  disintegration  and  by  a  process  akin  to  evapora- 
tion, so  that  the  surface  changes  its  appearance,  the  re- 
sistance increases  so  that  less  current  flows,  the  efficiency 
consequently  falls  off,  and  the  globe  shows  more  or  less 
blackening  from  an  internal  deposit  of  carbon. 

The  thinner  and  hotter  the  filament  the  less  its  en- 
durance and  the  sooner  it  deteriorates  or  actually  breaks 
down.  Modern  carbons  have  by  improved  methods  of 
manufacture  been  developed  to  a  point  that  in  the  early 
days  of  incandescent  lighting  would  have  seemed  be- 
yond hope  of  reach.  But  the  working  voltage  has 
steadily  risen  and  constantly  increased  the  difficulties  of 
the  manufacturer. 

So-called  high  efficiency  lamps  worked  at  about  3 
watts  per  candle  power  require  the  temperature  of  the 
filament  to  be  carried  so  high  that  its  life  is  seriously 
endangered  unless  it  be  of  fair  diameter;  hence  such 
lamps  are  hard  to  make  for  low  candle-power  or  for 
high  voltage,  either  of  which  conditions  requires  a 
slender  filament — in  the  former  case  to  limit  the  radiant 
surface,  in  the  latter  to  get  in  the  needful  resistance. 
An  8-cp  125-volt  lamp,  or  a  i6-cp  250- volt  lamp  pre- 
sents serious  difficulties  if  the  efficiency  must  be  high, 
while  conversely  lamps  of  24  or  32  candle-power  are  far 
more  easily  made  for  high  voltage. 


THE  ELECTRIC   INCANDESCENT   LAMP,     in 


The  annexed  table  gives  a  clear  idea  of  the  perform- 
ance of  a  modern  lamp  under  various  conditions  of  work- 
ing. It  is  from  tests  made  on  a  i6-cp  loo-volt  lamp  (so- 
called)  by  Professor  H.  J.  Weber.  The  effective  radiat- 
ing surface  of  the  filament  in  this  lamp  was  o.  1 178  square 
inch,  so  that  the  intrinsic  brilliancy  was  over  250  candle- 
power  per  square  inch. 


AMPERES 

VOLTS 

WATTS 

CP. 

WATTS    PER    CP.    TEMPERATURE 

0.421 

77-IQ 

32.51 

2.99 

10.87 

I464°C. 

0-443 

80.89 

35.85 

4  13 

8.67 

1483 

0.467 

84.80 

39-58 

5.6o 

7.07 

1503 

0.490 

88.83 

43-55 

7-41 

5-88 

1522 

0.513 

92.87 

47-70 

9.71 

4.91 

1541 

0.536 

96.71 

51-84 

12.42 

4.18 

1557 

0-559 

100  60 

56.21 

15.76 

3-57 

1574 

0.582 

104.58 

60.90 

19.70 

3-09 

1591 

o  605 

108.60 

65.78 

24.25 

2.71 

1607 

0.629 

112.57 

70.85 

29.41 

2.41 

1621 

The  absolute  values  of  the  temperatures  here  given 
are  the  least  exact  part  of  the  table,  but  the  relative 
values  may  be  trusted  to  a  close  approximation.  Fig.  35 
shows  in  graphical  form  the  relation  between  the  last 
two  columns,  showing  clearly  how  conspicuously  the 
efficiency  rises  with  the  temperature.  At  the  upper  limit 
given  the  carbon  is  too  hot  to  give  a  long  life,  although 
the  writer  has  seen  modern  lamps  worked  12  volts  above 
their  rating  for  several  hundred  hours  before  rapid 
breakage  began.  Of  course  the  brilliancy  had  fallen  off 
greatly,  however,  by  that  time. 

It  is  worth  noting  from  the  table  that  for  a  i6-cp  lamp 
of  ordinary  voltage  the  candle-power  varies  to  the  ex- 
tent of  quite  nearly  one  candle-power  per  volt,  for 
moderate  changes  of  voltage  from  the  normal.  Weber 
calls  attention  to  the  fact  that  between  1400  degrees  and 


112 


THE   ART   OF   ILLUMINATION. 


1650  degrees  an  increase  in  temperature  of  n  degrees 
corresponds  very  closely  to  a  saving  in  energy  of  n  per 
cent,  in  the  production  of  light. 

If  it  were  possible  to  carry  the  temperature  still  higher 
without  seriously  imparing  the  stability  of  the  filament, 
lamps  of  a  very  high  economy  could  be  produced.  It  is 
possible  to  force  lamps  up  to  an  economy  of  even  1.5 
watts  per  candle  temporarily,  but  they  often  break  al- 


5  6  7  8  9  10  II 

Watts  per  mean  horizontal  candle  power 

Fig.  35' — Variation  of  Efficiency  with  Temperature. 

most  at  once,  and  even  if  they  hold  together  they  rise  to 
2  or  2.5  watts  per  candle  within  a  few  hours. 

To  tell  the  truth,  the  temperature  corresponding  to 
1.5  watts  per  candle  is  dangerously  near  the  boiling 
point  of  the  material,  so  near  that  it  is  practically  hope- 
less to  expect  any  approximation  to  such  efficiency  from 
carbon  filaments,  and  even  at  2.5  watts  per  candle  the 
life  of  the  lamps  is  so  short  that  at  present  prices  they 
cannot  be  used  commercially. 

From  such  experiments  as  those  tabulated  it  has  been 
shown  that  the  relation  between  the  luminous  intensity 
and  the  energy  expended  in  an  incandescent  lamp  may 
be  expressed  quite  nearly  by  the  following  formula: 


THE  ELECTRIC   INCANDESCENT   LAMP.     113 

wherein  /  is  the  candle-power,  W  the  watts  used,  and  a 
is  a  quantity  approximately  constant  for  a  given  type 
of  lamp,  but  varying  slightly  from  type  to  type. 

Following  the  universal  rule  of  incandescent  bodies, 
the  radiation  from  an  incandescent  lamp  varies  in  color 
with  the  temperature,  and  thus  as  the  voltage  changes, 
or  what  is  about  the  same  thing,  as  lamps  of  different 
efficiencies  are  used,  the  color  of  the  light  varies  very 
conspicuously.  Low  efficiency  lamps,  or  lamps  in  a  low 
stage  of  incandescence,  such  as  is  indicated  in  the  first 
four  lines  of  the  table,  burn  distinctly  red  or  reddish 
orange.  Then  the  incandescence  passes  through  the 
various  stages  of  orange  yellow  and  yellow  white  until 
a  3- watt  lamp  is  nearly  and  a  2.5  watt  lamp  purely  and 
dazzlingly,  white.  The  color  is  a  good  index  of  the 
efficiency. 

The  sizes  of  incandescent  lamps  in  fairly  common  use 
are  8,  10,  16,  20,  24,  and  32  candle-power.  The 
standard  in  this  country  is  the  i6-cp  size,  a  figure  bor- 
rowed from  the  legal  requirements  for  gas.  Some  10 
candle-power  are  used  here,  very  few  8  candle-power, 
and  still  fewer  of  candle-powers  above  16.  Abroad  8-cp 
lamps  are  used  in  great  numbers  and  with  excellent  re- 
results.  The  2O-cp  and  24-cp  lamps  are  found  mostly  in 
high  voltages,  for  reasons  that  will  appear  shortly.  Four 
and  6-cp  lamps  are  now  and  then  used  for  decorative 
purposes  or  for  night-lights,  and  excellent  5O-cp  lamps 
are  available  for  cases  requiring  radiants  of  unusual 
power. 

Lamps  of  these  various  sizes  are  made  usually  for  volt- 
ages between  100  and  120  volts,  and  more  rarely  for 
220  to  250  volts,  but  in  the  latter  case  lamps  below  16 
candle-power  are  almost  unknown  in  America. 


ii4  THE   ART    OF   ILLUMINATION. 

One  hundred  and  ten  volts  was  for  some  years  the 
standard  pressure  here,  and  with  this  as  a  basis  one  may 
profitably  see  what  are  the  problems  to  be  met  in  lamp 
construction.  At  this  voltage  the  filament  of  a  i6-cp 
lamp  is  6  or  8  inches  long  and  .008  to  .01  inch  in 
diameter,  and  ordinarily  has  a  resistance  when  hot  of 
nearly  200  ohms.  Now  to  produce  an  8-cp  lamp  of  the 
same  voltage  and  efficiency  the  energy  consumed  must 
be  reduced  by  one-half,  and  so  also  must  be  the  radiating 
surface.  This  means  that  the  filament  resistance  must 
be  doubled,  and  the  radiating  surface  so  adjusted  by 
varying  the  length,  diameter,  and  specific  resistance  as 
to  give  the  required  candle-power. 

The  latter  two  factors  can  be  varied  during  the  process 
of  flashing,  since  the  carbon  deposited  thus  is  denser 
and  of  lower  specific  resistance  than  the  original 
squirted  core.  The  net  result  is  a  filament  consider- 
ably slenderer  than  the  i6-cp  filament  and  usually 
of  less  stability.  On  the  other  hand,  in  making  a 
32-cp  lamp  the  filament  may  conveniently  be  made 
longer,  thicker,  and  more  durable.  In  lamps  of  higher 
voltage  the  filaments  must  be  of  much  higher  resistance, 
and  hence  longer  and  thinner,  until  at  220  volts  the 
i6-cp  lamp  must  have  four  times  the  resistance  of  its 
1 10- volt  progenitor,  and  commonly  has  a  total  length  of 
filament  of  12  to  15  inches. 

In  lamps  of  small  candle-power  or  of  high  voltage 
there  is  some  temptation  to  get  resistance  by  flashing 
the  filaments  less  thoroughly,  to  the  detriment  of  dura- 
bility, since  the  soft  core  disintegrates  more  readily  than 
the  hard  deposited  carbon,  which  may  explain  the  fre- 
quent inferiority  of  such  lamps.  The  greater  the  candle- 
power,  and  the  less  efficiency  required,  i.  e.,  the  greater 


THE  ELECTRIC  INCANDESCENT  LAMP.     115 

the  permissible  radiating  surface,  the  easier  it  is  to  get 
a  strong  and  durable  filament  for  high  voltages.  Hence, 
lamps  for  220  to  250  volts  are  generally  of  at  least  16 
candle-power,  very  often  of  20  or  24  candle-power,  and 
seldom  show  an  efficiency  better  than  4  watts  per  candle- 
power. 

This  forms  a  serious  practical  objection  to  the  use  of 
such  lamps  for  general  distribution,  unless  with  cheap 
water-power  as  the  source  of  energy,  ajnd  while  im- 
proved methods  of  manufacture  are  likely  somewhat  to 
better  these  conditions,  yet  there  are  inherent  reasons 
why  it  should  be  materially  easier  to  produce  durable 
and  efficient  incandescent  lamps  of  moderate  candle- 
power  and  voltage  than  lamps  of  extreme  properties  in 
either  of  these  directions. 

The  life  of  incandescent  lamps  practically  depends  on 
the  temperature  at  which  they  are  worked,  other  things 
being  equal.  There  is  a  steady  vaporization  and  disin- 
tegration of  the  carbon  from  the  moment  the  lamp  is 
put  into  service,  which  ends  in  a  material  increase  in  the 
resistance  of  the  filament  with  accompanying  decrease 
of  the  current,  energy,  temperature,  efficiency,  and 
light. 

If  the  lamp  is  started  at  a  low  efficiency  the  tem- 
perature is  relatively  low  and  the  decadence  of  the  fila- 
ment is  retarded,  while  if  the  lamp  is  initially  of  high 
efficiency  the  filament  under  the  higher  temperature  de- 
teriorates more  rapidly  and  the  useful  life  of  the  lamp 
is  shortened. 

Under  this  latter  condition  the  cost  of  energy  to  run 
the  lamp  is  diminished,  but  at  the  price  of  increased  ex- 
pense in  lamp  renewals.  Operating  at  low  efficiency 
means  considerable  cost  for  energy  and  low  cost  of  the 


n6  THE   ART   OF   ILLUMINATION. 

lamp  renewals.  Between  these  divergent  factors  an  eco- 
nomic balance  has  to  be  struck. 

It  is  neither  desirable  nor  economical  to  operate  an 
incandescent  lamp  too  long,  since  not  only  does  it  de- 
crease greatly  in  efficiency,  but  the  actual  light  is  so 
dimmed  that  the  service  becomes  poor.  If  the  lighting 
of  a  room  is  planned  for  the  use  of  i6-cp  lamps,  and  they 
are  used  until  the  candle-power  falls  to,  say,  10,  which 
would  be  in  about  600  hours  in  an  ordinary  3-watt-per- 
candle  lamp,  the  resulting  illumination  would  be  alto- 
gether unsatisfactory.  Quite  aside  from  any  considera- 
tion of  efficiency,  therefore,  it  becomes  desirable  to 
throw  away  lamps  of  which  the  candle-power  has  fallen 
below  a  certain  point. 

Much  of  the  skill  in  modern  lamp  manufacture  is  di- 
rected to  securing  the  best  possible  balance  between 
efficiency  and  useful  life,  a  thing  requiring  the  best  ef- 
forts of  the  manufacturer.  Fig.  36  shows  graphically  the 
relation  between  life,  candle-power,  and  watts  per  candle 
derived  from  tests  of  high-grade  foreign  lamps.  In  com- 
paring these,  like  the  previous  data,  with  American  re- 
sults, it  should  be  borne  in  mind  that  these  foreign  tests 
are  made,  not  in  terms  of  the  English  standard  candle, 
but  generally  in  terms  of  the  Hefner-Alteneck  standard, 
which  is  somewhat  (approximately  10  per  cent.)  smaller. 

These  curves  show  the  results  from  lamps  having  an 
initial  efficiency  of  2.5,  3.0,  and  3.5  watts  per  candle- 
power  and  an  initial  candle-power  of  16.  They  show 
plainly  the  effect  of  increased  temperature  on  the  life 
of  the  lamp,  and  it  is  unpleasantly  evident  that  in  the 
neighborhood  of  3  watts  per  candle  a  point  is  reached 
at  which  a  further  increase  of  efficiency  produces  a  dis- 
astrous result  upon  the  life.  In  other  words,  that  ef- 


THE  ELECTRIC  INCANDESCENT  LAMP.     117 

ficiency  requires  a  temperature  at  which   the  carbon 
filament  rapidly  breaks  down. 

And  so  long  as  carbon  is  used  as  the  radiant  material 
there  is  a  strong  probability  that  there  can  be  no  very 
radical  improvement  in  efficiency.  Of  course,  if  incan- 


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Curves  a=watts  per  c,p.         Curves  b=c.p, 

Fig.  36.— Curves  Showing  Life,  Candle-power  and  Watts  per 

Candle. 

descent  lamps  were  greatly  cheapened,  it  would  pay  to 
burn  them  at  a  higher  efficiency  and  to  replace  them 
oftener.  It  is  quite  possible  that  increased  experience 
and  persistent  efforts  at  standardization  might  lead  to 
this  result. 

In  production  on  a  large  scale  the  mere  manufacture  of 
the  lamps  can  be  done  very  cheaply,  probably  at  a  cost 
not  exceeding  7  to  8  cents,  but  the  cost  of  proper  sorting 
and  testing  to  turn  out  a  uniform  high-grade  lamp,  and 
the  incidental  losses  from  breakage  and  from  lamps  of 
odd  and  unsalable  voltages,  raises  the  total  cost  of  produc- 
tion very  materially.  Much  of  the  reduction  in  the  price 
of  incandescent  lamps  in.  the  past  few  years  has  resulted 
from  better  conditions  in  these  latter  respects,  as  well 
as  from  the  improved  methods  of  manufacture. 


n8  THE   ART   OF   ILLUMINATION. 

And  it  should  be  pointed  out  that  the  difference  be- 
tween good  and  bad  lamps,  as  practically  found  upon 
the  market,  lies  mostly  in  their  different  rates  of  decay 
of  light  and  efficiency.  It  is  the  practice  of  many  of  the 
large  lighting  companies  who  renew  the  lamps  for  which 
they  furnish  current  to  reject  and  replace  lamps  which 
have  fallen  to  about  80  per  cent,  of  their  initial 
power. 

First-class  modern  lamps  worked  in  the  vicinity  of  3 
watts  per  candle-power  will  hold  up  for  400  to  450  hours 
before  falling  below  this  limit,  and  at  3.5  or  3.6  watts  per 
candle-power  will  endure  nearly  double  that  time. 
They  are  often  rated  in  candle-hours  of  effective  life,  and 
on  the  showing  just  noted  the  recent  high  efficiency 
lamp  will  give  a  useful  life  of  6500  to  7000  candle-hours, 
with  an  average  economy  of  perhaps  3.25  watts  per 
candle.  A  medium  grade  lamp  of  similar  nominal  ef- 
ficiency may  not  show  with  a  similar  consumption  of 
energy  more  than  250  or  300  hours  of  effective  life — say 
4000  to  4500  candle-hours. 

The  economics  of  the  matter  appear  as  follows:  The 
first  lamp  during  its  useful  life  of,  say  6500  candle-hours, 
will  consume  21.125  kilowatt-hours,  costing  at,  say,  15 
cents  per  kilowatt-hour,  $3.17,  and  adding  the  lamp  at  18 
cents,  the  total  cost  is  $3.35,  or  0.0515  cent  per  candle- 
hour,  while  the  poorer  lamp  at  4000  candle-hours  will 
use  $1.95  worth  of  energy,  and  at  18  cents  for  the  lamp, 
would  cost  0.0532  cent  per  candle-hour.  To  bring  the 
two  lamps  to  equality  of  total  cost,  irrespective  of  the 
labor  of  renewals,  the  poorer  one  would  have  to  be  pur- 
chased at  1 1  cents.  In  other  words,  poor  lamps,  if  dis- 
carded when  they  should  be,  generally  so  increase  the 
cost  of  renewals  that  it  does  not  pay  to  use  them  at  any 


THE   ELECTRIC   INCANDESCENT   LAMP.     119 

price  at  which  they  can  be  purchased  under  ordinary 
circumstances. 

As  has  already  been  explained,  lamps  deteriorate  very 
rapidly  if  exposed  to  abnormal  voltage,  and  the  higher 
the  temperature  at  which  the  lamp  is  normally  worked  the 
more  deadly  is  the  effect  of  increased  voltage.  It  thus 
comes  about  that  if  high  efficiency  lamps  are  to  be  used, 
very  good  regulation  is  necessary.  Occasional  exposure 
to  a  5  per  cent,  increase  of  voltage  may  easily  halve  the 
useful  life  of  a  lamp,  while,  of  course,  permanent  work- 
ing at  such  an  increase  would  play  havoc  with  the  life, 
cutting  it  down  to  20  per  cent,  or  less  of  the  normal. 
Good  regulation  is  therefore  of  very  great  importance 
in  incandescent  lighting,  not  only  to  save  the  lamps  and 
to  improve  the  service,  but  to  render  feasible  the  use  of 
high  efficiency  lamps.  On  the  whole,  the  best  average 
results  seem  to  be  obtained  in  working  lamps  at  3  to  3.5 
watts  per  candle.  Those  of  higher  efficiency  fail  so 
rapidly  that  it  only  pays  to  use  them  when  energy  is  very 
expensive  and  must  be  economized  to  the  utmost.  The 
2.5-watt  lamp  of  Fig.  36,  for  example,  has  an  effective 
life  of  not  more  than  1 50  hours,  at  an  average  efficiency 
of  about  2.75  watts  per  candle.  A  2-watt  lamp  will  fall 
to  80  per  cent,  of  its  original  candle-power  in  not  far 
from  30  hours,  at  an  average  efficiency  of  about  2.25 
watts,  while  if  started  as  a  i.5-watt  lamp,  in  a  few  hours 
the  filament  is  reduced  to  practical  uselessness. 

There  is  seldom  any  occasion  to  use  lamps  requiring 
more  than  3.5  watts  per  candle-power,  save  in  case  of 
very  high  voltage  installations,  where  the  saving  in  cost 
of  distribution  may  offset  the  cost  of  the  added  energy. 
The  difficulty  of  making  durable  25o-volt  lamps  on  ac- 
count of  the  extreme  thinness  of  the  filament  has  been 


120  THE   ART   OF   ILLUMINATION. 

already  referred  to,  and  it  is  certainly  advisable  to  use 
in  such  installations  lamps  of  20  candle-power  or  more 
whenever  possible,  thus  making  it  practicable  to  work  at 
better  efficiency  without  increased  risk  of  breakage. 
Even  when  power  is  very  cheap  there  is  no  object  in 
wasting  it,  and  a  little  care  will  generally  procure  regu- 
lation good  enough  to  justify  the  employment  of  in- 
candescent lamps  of  good  efficiency. 

Further,  in  the  commercial  use  of  lamps  it  is  necessary 
for  economy  that  the  product  should  be  uniform.  It 
has  already  been  shown  that  medium  grade  lamps  are 
characterized  by  a  shorter  useful  life  than  first-class 
lamps.  Unfortunately,  there  are  on  the  market  much 
worse  lamps  than  those  described.  It  is  not  difficult  to 
find  lamps  in  quantity  that  are  so  poor  as  to  fall  to  80  per 
cent,  of  their  initial  power  in  less  than  100  hours.  A 
brief  computation  of  the  cost  of  replacement  will  show 
that  these  are  dear  at  any  price.  Now,  if  lamps  are  not 
carefully  sorted,  a  given  lot  will  contain  both  good 
lamps  and  poor  lamps,  and  will  not  only  show  a  de- 
creased average  value,  but  will  contain  many  individual 
lamps  so  bad  as  to  give  very  poor  and  uneconomical 
service.  Fig.  37  shows  what  is  sometimes  known  as  a 
"  shotgun  diagram,"  illustrating  the  variations  found  in 
carelessly  sorted  commercial  lamps.  In  this  case  the 
specifications  called  for  i6-cp,  3-5-watt  per  candle-power 
lamps.  The  variation  permitted  was  from  14.5  to  17.5 
mean  horizontal  candle-power,  and  from  53  to  59  total 
watts,  which  is  a  liberal  allowance,  some  companies  de- 
manding a  decidedly  closer  adherence  to  the  specified 
limits. 

The  area  defined  by  these  limits  is  marked  off  in  the 
cut,  forming  the  central  "  target."     The  real  measure- 


THE  ELECTRIC   INCANDESCENT   LAMP.     121 


ments  of  the  lamps  tested  are  then  plotted  on  the  dia- 
gram and  the  briefest  inspection  shows  the  results.  In 
this  case  only  46  per  cent,  of  the  lamps  hit  the  specifica- 


52      53       54 


17 


SI.SW. 


13 

50.1  W. 


Watts 
55       56 


57       58       59 


sow. 


60.7  W. 


62.8  W. 


Fig.  37. — Shotgun  Diagram. 

tions.  All  lamps  above  the  upper  slanting  line  are  be- 
low 3.1  watts  per  candle-power,  and  hence  are  likely 
to  give  trouble  by  falling  rapidly  in  brilliancy  and  break- 
ing early.  Lamps  below  the  lower  slanting  line  are  over 
4  watts  per  candle-power,  hence  are  undesirably  inef- 
ficient. Moreover,  the  initial  candle-power  of  the  lot 
varies  from  12.2  candle-power  to  20.4  candle-power. 


122  THE    ART    OF    ILLUMINATION. 

Such  a  lot  will  necessarily  give  poorer  service  and  less 
satisfactory  life,  and  is,  as  a  matter  of  dollars  and  cents, 
worth  much  less  to  the  user  than  if  the  lamps  had  been 
properly  sorted  at  the  factory.  Filaments  cannot  be 
made  exactly  alike,  and  the  manufacturer  has  to  rely 
upon  intelligent  sorting  to  make  use  of  the  product. 
For  example,  the  topmost  lamp  of  Fig.  37  should  have 
been  marked  for  a  lower  voltage,  at  which  it  would  have 
done  well.  Nearly  all  the  lot  would  have  properly  fallen 
within  commercial  specifications  for  i6-cp  lamps  at 
some  practicable  voltage  and  rating  in  watts  per  candle- 
power.  The  imperfect  sorting  has  misplaced  many  of 
the  lamps  and  depreciated  the  whole  lot. 

In  commercial  practice  lamps  should  be  carefully 
sorted  to  meet  the  required  specifications,  and  the  per- 
sons who  buy  lamps  should  insist  upon  rigid  adherence 
to  the  specifications  and  should,  in  buying  large  quan- 
tities, test  them  to  ensure  their  correctness.  To  sum 
up,  it  pays  to  use  good  lamps  of  as  high  efficiency  as  is 
compatible  with  proper  life,  and  to  see  that  one  gets 
them. 

The  real  efficiency  of  an  incandescent  lamp,  i.  e.,  the 
proportion  of  the  total  energy  supplied  which  appears  as 
visible  luminous  energy,  is  very  small,  ordinarily  from 
4  to  6  per  cent.,  not  over  6.5  per  cent,  even  in  a  3-watt 
per  candle-power  lamp.  This  means  that  in  working 
incandescent  lamps  from  steam-driven  plants  not  over 
0.5  per  cent,  of  the  energy  of  the  coal  appears  as  useful 
light.  This  is  a  sad  showing,  and  one  which  should  spur 
invention.  To  get  better  results,  it  seems  necessary  to 
abandon  the  carbon  filament,  at  least  in  any  form  in 
which  we  now  know  it,  and  either  to  turn  to  some  other 
material  for  the  incandescent  body,  or  to  abandon  the 


THE   ELECTRIC   INCANDESCENT   LAMP.     123 

principle  of  incandescence  altogether  and  pass  to  some 
form  of  lamp  in  which  the  luminosity  is  not  due  to  the 
high  temperature  of  a  solid  radiant.  The  writer  is 
strongly  disposed  to  think  that  the  ultimate  solution 
lies  in  the  latter  alternative,  although  the  former  offers 
hope  of  very  considerable  and  perhaps  revolutionary 
improvements. 

Within  the  past  few  years  a  large  number  of  attempts 
have  been  made  at  preparing  a  filament  for  incandescent 
lamps  of  some  material  far  more  refractory  than  pure 
carbon,  and  hence  better  able  to  endure  the  high  tem- 
perature necessary  for  securing  high  efficiency.  A 
glance  at  the  temperature  curve,  Fig.  35,  shows  that  a 
rise  of  200  degrees  C.  or  so  in  the  working  temperature 
would  produce  an  efficiency  of  nearly  or  quite  one  candle- 
power  per  watt. 

These  attempts  have  been  of  several  kinds.  One 
method  has  been  to  incorporate  refractory  material  with 
the  carbon  in  manufacturing  the  filaments,  thus  both  in- 
creasing the  resistance  of  the  filaments  and  giving  them  a 
certain  proportion  of  heat-resisting  substances.  Owing, 
however,  to  the  fact  that  such  filaments  still  contain  a 
considerable  proportion  of  carbon  which  is  compara- 
tively easily  vaporized,  there  is  good  reason  to  doubt 
the  efficacy  of  the  process.  The  carbon,  which  is  the 
cement,  as  it  were,  once  disintegrated,  the  filament 
would  give  way,  and  experience  up  to  date  has  tended 
to  throw  doubt  on  the  success  of  any  such  scheme. 

An  interesting  modification  of  this  method  is  that  pro- 
posed by  Langhans,  who  forms  filaments  of  carbide  of 
silicon,  i.  e.,  employs  carbon  in  chemical  combination 
instead  of  merely  as  a  species  of  cement.  This  process 
has  not  been  carried  to  commercial  success,  but  it  cer- 


124  THE   ART   OF   ILLUMINATION. 

tainly  looks  more  hopeful,  on  general  principles,  than  the 
process  of  incorporation. 

Another  line  of  attack  on  the  problem  is  that  of  Auer 
von  Welsbach,  who  proposed  a  filament  of  platinum  or 
similar  metal,  coated  with  thoria,  the  rare  earth  which 
is  the  chief  constituent  of  the  Welsbach  mantle.  This 
looks  mechanically  dubious.  Still  another  modification 
of  this  idea  is  the  use  of  a  filament  mainly  of  carbon,  but 
with  a  coherent  coating  of  thoria  or  the  like,  a  line  of 
investigation  which  appears  worth  pursuing.  Akin  to 
this  is  the  Nernst  lamp,  which  is  at  present  exciting  great 
interest,  although  it  is  barely  yet  in  the  commercial 
stage.  The  basic  fact  on  which  Dr.  Nernst's  work  is 
founded  is  that  many  substances,  non-conductors  at 
ordinary  temperatures,  become  fairly  good  conductors 
when  heated.  Thus  a  tiny  pencil  of  lime,  magnesia,  or 
the  rare  earths,  when  once  heated,  will  allow  a  current  to 
pass  at  commercial  voltages  sufficient  to  maintain  it  at 
vivid  incandescence.  From  this  fundamental  fact 
Nernst  has  developed  a  most  interesting  and  promising 
glow  lamp. 

The  variation  of  resistance  with  temperature  in  such 
substances  as  the  rare  earths  as  used  by  Nernst  is  truly 
prodigious.  They  seem  really  to  pass  from  insulators  to 
conductors.  Even  glass,  in  fact,  conducts  fairly  well  at 
high  temperatures,  although  in  all  such  cases  conduction 
is  probably,  at  least  in  part,  electrolytic  in  its  character, 
a  fact  which  is  of  considerable  practical  moment.  As 
developed  by  Nernst  the  filaments  when  cold  have  sev- 
eral hundred  times  the  resistance  which  they  have  when 
hot.  Fig.  38  shows  graphically  from  Nernst's  tests  the 
way  in  which  the  specific  resistance  falls  off  as  the  tem- 
perature rises.  From  the  somewhat  meager  data  it  is 


THE  ELECTRIC   INCANDESCENT   LAMP.     125 

of  necessity  only  approximate,  but  it  gives  a  vivid  idea 
of  the  extraordinary  nature  of  the  phenomenon.  Of 
course,  carbon  shows  a  great  decrease  of  resistance  when 
hot,  but  it  is  a  pretty  fair  conductor  when  cold,  while 
the  Nernst  filament  is  practically  an  insulator  in  that 


1100 


500 


2000 
Ohms  per  Cubic  Centimeter 


4000 


Fig.  38. — Curve  of  Resistance  Variation. 

condition.  But  the  oxides  of  the  Nernst  filament  are 
enormously  more  refractory  than  carbon,  and  can  not 
only  be  carried  to  far  higher  incandescence  without 
breaking  down,  but  probably  have,  at  least  in  some  of 
the  combinations  used,  a  rather  more  efficient  distribu- 
tion of  energy  in  the  spectrum  than  is  the  case  with 
carbon. 

But  being  an  insulator  at  ordinary  temperatures  some 
means  has  to  be  taken  to  get  current  through  the  fila- 
ment. It  has  long  been  known  in  a  general  way  that 
magnesia  and  similar  materials  conduct  at  a  high  tern- 


126 


THE   ART   OF   ILLUMINATION. 


perature,  and  both  Le  Roux  and  Jablochkoff  had  dab- 
bled with  the  idea  years  ago.  But  Nernst  took  up  the 
matter  anew  and  in  earnest.  The  lamp  which  he  has 
produced  consists  essentially  of  a  thin  pencil  of  mixed 
oxides,  forming  the  incandescent  body.  This  pencil  is 
much  thicker  and  shorter  than  a  carbon  filament  as  used 


Metdl 


Fig.  39  and  40. — Connections  of  Nernst  Lamp. 

in  incandescent  lamps,  being,  say,  from  1-64  to  1-16  inch 
in  diameter,  and  3-4  to  i  1-2  inch  long.  If  heated  by 
a  match  or  spirit  lamp  the  filament  becomes  a  conductor, 
and  goes  to  vivid  incandescence.  Such  artificial  heating 
being  somewhat  of  a  nuisance,  much  of  the  work  spent 
in  developing  the  Nernst  lamp  has  been  in  the  direction 
of  providing  means  for  artificial  lighting.  As  developed 
abroad  the  self-lighting  Nernst  lamp  has  taken  the  form 
shown  in  Fig.  39.  Rising  from  the  base  of  the  lamp  G 


THE  ELECTRIC   INCANDESCENT  LAMP.     127 

are  two  stiff  wires,  DD,  spaced  near  the  ends  by  a  porce- 
lain disk  C.  Across  the  platinum  tips  of  these  rods  is 
fastened  the  glower  A,  secured  at  its  terminals  by  con- 
ducting cement.  Coiled  in  loose  turns  about  the  glower 
is  a  porcelain  spiral  B,  into  the  surface  of  which  has 
been  baked  a  fine  platinum  wire  closely  coiled  around 
it.  The  office  of  this  resistance  spiral  is  to  bring  the 
glower  to  a  temperature  at  which  it  begins  to  conduct. 
At  the  start  A  and  B  are  in  shunt,  but  when  current  gets 
fairly  started  through  the  former  it  energizes  a  tiny 
electro-magnet,  F,  situated  in  the  base  of  the  lamp,  its 
armature  L  is  attracted  and  the  circuit  through  B  broken, 
turning  the  whole  current  through  A. 

At  £  is  a  very  interesting  and  important  feature  of  the 
lamp.  It  is  a  "  ballast  "  resistance  of  fine  iron  wire 
wound  upon  a  porcelain  rod  and  sealed  into  a  little  bulb 
to  prevent  oxidation.  It  is  connected  in  series  with  the 
filament.  Now  iron  has  a  resistance  that  increases  rap- 
idly with  the  temperature,  and  this  increase  is  particu- 
larly rapid  at  about  450  to  500  degrees  C.  This  re- 
sistance coil  is  designed  so  that  with  normal  current  in 
the  lamp  the  temperature  will  rise  to  the  point  noted, 
and  its  office  is  to  steady  the  lamp.  Without  it  the 
Nernst  lamp  would  be  terribly  sensitive  to  variations  in 
voltage,  but  if  the  voltage  rises  with  this  resistance  in 
circuit,  its  increasing  resistance  chokes  the  current. 
Even  with  this  steadying  element  the  Nernst  lamp  is  still 
somewhat  sensitive  to  changes  of  voltage.  The  glower 
does  not  function  properly  in  an  exhausted  globe,  and 
must  be  worked  in  the  free  air,  although  a  glass  shade 
is  provided  to  protect  it  from  draughts,  dust,  etc.  In 
point  of  fact,  protection  from  draughts  is  at  present 
rather  necessary,  since  the  filament  is  so  sensitive  to 


128 


THE   ART   OF   ILLUMINATION. 


changes'  in  temperature  that  it  can  readily  be  blown  out 
by  the  breath.  Fig.  40  gives  a  clear  idea  of  the  con- 
nections of  the  lamp  of  Fig.  39,  while  Fig.  41  shows 


Fig.  41.  Nernst  Lamps. 

complete  at  the  left,  one  of  the  earlier  Nernst  lamps 
without  the  automatic  lighting  device. 

The  foreign  manufacturers  of  these  lamps  are  pro- 
ducing them  of  25,  50,  and  100  candle-power,  for  no- 
and  22O-volt  circuits.  The  Nernst  principle  lends  itself 
more  readily  to  powerful  high-voltage  lamps  than  to 
small  low  voltage  ones,  and  the  glower  is  found  to  work 
better  on  alternating  than  on  continuous-current  circuits, 


THE   ELECTRIC   INCANDESCENT  LAMP.     129 

apparently  for  reasons  depending  on  the  electrolytic  na- 
ture of  the  conduction.  Like  other  incandescent  lamps,  it 
works  the  more  efficiently  as  the  temperature  rises,  but 
owing  to  its  refractory  composition  the  Nernst  filament 
can  be  pushed  to  very  high  efficiency.  As  the  lamps  are 
produced  at  the  present  time  their  initial  efficiency  is 
about  1.50  to  1.75  watts  per  candle,  including  the  energy 
lost  in  the  steadying  resistance  (about  10  per  cent.),  and 
the  useful  life  is  said  to  be  about  300  hours.  The  fila- 
ment at  the  end  of  this  time  rises  in  resistance  and  falls 
in  efficiency,  much  like  an  ordinary  incandescent  fila- 
ment, but  rather  more  suddenly. 

The  real  life  of  a  Nernst  lamp,  when  defined  as  it 
should  be  in  terms  of,  say,  a  2O-per-cent.  drop  in  candle- 
power,  is  not  at  the  present  time  known.  It  has  only 
been  put  upon  the  market  abroad  within  the  present 
year,  and  the  owners  of  the  American  rights  have  not 
yet  put  lamps  out  in  large  commercial  quantities,  so  that 
really  accurate  data  are  entirely  lacking  as  yet. 

The  American  Nernst  lamp,  as  developed  in  the 
Westinghouse  laboratory,  retains  all  the  general  features 
of  the  foreign  lamps,  but  is  modified  in  some  important 
details.  It  has  been  so  designed  as  to  give  a  considerably 
longer  life  at  a  slightly  lower  efficiency.  The  heaters  are 
good-sized  simple  cylinders  of  porcelain,  instead  of 
spirals,  and  are  placed  close  to  and  above  the  glower. 
The  unit  is  a  single  5O-cp  glower,  but  it  is  found  that 
from  the  higher  working  temperature  and  better  con- 
servation of  heat  in  a  multiple  glower  lamp  a  better 
efficiency  is  obtained,  so  that  the  ordinary  sizes  are  those 
with  two,  three,  and  six  glowers,  rated  respectively  at 
100,  170,  and  400  candle-power.  This  rating  is  in  the 
direction  of  maximum  intensity. 


THE   ART   OF   ILLUMINATION. 


Fig.  42  shows  the  heaters  and  glowers  of  these  lamps 
assembled  on  a  porcelain  cap  with  connection  wires 
which  automatically  make  all  the  necessary  connections 
when  the  holder  is  pushed  into  its  base.  Thus  far  only 
the  single  glower  lamp  is  made  for  no  volts,  the  others 
being  for  220  volts.  Fig.  43  shows  the  general  appear- 
ance of  the  lamps  as  fitted  for  indoor  use. 


MEAN  INTENSITY  IN  H.  U. 

WATTS  PER  MEAN  H.  U. 

Spherical 

Lower 
Hemisphere 

Spherical 

Lower 
Hemisphere 

R 

I 

£ 

0) 

K 

I* 

W 

1 

§ 

B 

o 

tj 

fa 

O 

O 

'ea 

O 

O 

1 

i 

O 

•o 

0 

O      ,   "° 

o  . 

i 

^ 

u 

j 

IH 
O 

w 

"5 
a. 

8 

1 

« 

M 

1 

rt 

M 

£ 

0 

0 

o 

O 

o 

0 

U 

f6-Glower 

220 

2.35 

517 

I.O 

149* 



147 

240* 

— 

279 

347* 

—  !3-5 

2.15* 



1.85 

A.  C.  Arc. 

no 

5.29 

417 

.6 

130 

159 

152 

— 

190 

2543  21 

2.62  2.49 

— 

2.23  11.48 

D.  C.  Arc 

no 

4.9 

539 

I.O 

177 

207 

— 

— 

272 

— 

3-03 

2.60 

— 

— 

1.98 

— 

An  opal  inner  globe  or  heater-case  was  used  in  all  cases  except  the  four  readings 
marked.* 

*  A  clear  heater-case  and  sand-blasted  spherical  globe  were  used. 
f  Rated  at  400  cp. 

The  foregoing  table  gives  its  performance  as  com- 
pared with  alternating  current  and  direct  current 
enclosed  arc  lamps,  the  intensities  being  in  Hef- 
ner units.  From  this  it  appears  that  the  Nernst  lamp  is 
fairly  comparable  in  efficiency  with  the  enclosed  arcs, 
while  giving  a  steadier  light  decidedly  better  in  color. 

The  distribution  of  light  from  these  lamps  is  obviously 
somewhat  peculiar.  It  is  specially  strong  in  the  lower 
hemisphere,  being  designed  with  downward  illumina- 
tion in  mind.  The  horizontal  distribution  from  a  single 
glower,  as  determined  by  M.  Hospitalier,  is  shown  in 


THE  ELECTRIC  INCANDESCENT  LAMP.     131 

Fig.  44.  The  glower  was  horizontal  and  the  measure- 
ments were  taken  in  the  horizontal  plane  passing 
through  it.  The  section  of  the  glower  appears  in  the 
center  of  the  diagram.  Broadside  on  this  glower  gave 
about  40  cande-power;  when  nearly  end  on,  about  10 
candle-power. 

After  about  100  hours'  run  the  inner  globe  or  "  heater- 
case  "  becomes  darkened  by  a  deposit  from  the  glower 
and  its  platinum  contacts  and  from  the  heaters,  and  has 


Fig.  42. — Glowers  and  Heaters  of  American  Nernst  Lamp. 

to  be  cleansed,  so  that  with  respect  to  care  the  new  lamp 
resembles  arcs  rather  than  incandescents.  The  effective 
life  of  the  glowers  is  said  to  be  about  800  hours,  the  effi- 
ciency holding  up  pretty  well  until  they  break.  The  in- 
trinsic brilliancy  of  the  glower  is  very  great,  1000  to  1250 
cp  per  square  inch.  Hence  the  shading  of  Nernst  lamps 
by  diffusing  globes  or  other  screens  must  be  very  thor- 
ough, so  as  to  cut  down  the  intolerable  brightness  of  the 
glower  itself. 

The  automatic  lighting  device  seems  to  work  well, 
bringing  the  lamp  up  to  full  brilliancy  in  not  far  from 


132 


THE   ART   OF   ILLUMINATION. 


half  a  minute.  On  continuous  current  the  life  of  the 
glower  is  very  greatly  reduced,  probably  to  one-third 
its  normal  duration,  so  that  at  present  the  device  be- 
longs essentially  to  alternating  current  distributions, 
and  the  life  also  tends  to  increase  with  the  frequency, 


Fig.  43. — Types  of  ..ndoor  Lamps. 

so  that  the  very  low  frequency  circuits  are  somewhat  at 
a  disadvantage  in  using  Nernst  lamps. 

It  seems,  however,  certain  that  the  Nernst  lamp  is  an 
important  addition  to  the  art  within  at  least  a  limited 
sphere  of  usefulness. 

The  glower  can  be  replaced  at  a  moderate  cost,  ulti- 
mately below  the  cost  of  replacement  of  incandescent 
lamps  of  equivalent  candle-power,  so  that  even  with 
a  rather  short  life  of  the  glower  the  lamp  would  still 
be  economical  in  use.  While  less  efficient  than  the 
best  arc  lamps,  it  compares  favorably  with  enclosed  arcs 
of  moderate  amperage,  and  it  is  just  now  to  be  regarded 


THE  ELECTRIC  INCANDESCENT  LAMP.     133 

rather  as  a  competitor  of  the  arc  than  of  the  glow  lamp. 
However,  it  would  take  no  great  advance  to  change  this 
condition,  and  the  ease  with  which  Nernst  lamps  may 
be  made  for  high  voltage  is  a  rather  important  matter. 
If  one  institutes  a  comparison  on  the  basis  of  25o-volt 
lamps  the  result  is  very  greatly  in  favor  of  the  Nernst 


36  /  36 


Fig.  44. — Horizontal  Distribution  from  Nernst  Lamp. 

filament  at  any  reasonable  estimate  of  its  life.  As  in 
ordinary  lamps  so  with  Nernst  lamps,  filaments  for  small 
candle-power  involve  unusual  difficulties,  but  at  present 
the  i6-cp  lamp  should  be  taken  as  the  normal  glow 
lamp,  while  with  the  Nernst  lamp  perhaps  5o-cp  may  be 
regarded  as  the  normal  unit  glower. 

At  present  one  must  regard  the  Nernst  lamp  as  in  a 
tentative  condition,  and  various  problems  regarding  it 
must  be  threshed  out — in  particular  there  should  be 
radical  improvement  in  the  automatic  lighting  device  or 
such  evolution  of  a  filament  of  higher  initial  conductivity 
as  will  obviate  further  necessity  for  special  lighting  de- 
vices. But  a  highly  efficient  and  very  easily  replaced 
incandescent  body  is  in  itself  a  material  advantage  over 
the  delicate  filament  and  exhausted  globe  of  the  ordi- 
nary incandescent  lamp,  and  unless  the  Nernst  lamp  shall 


1 34  THE   ART    OF   ILLUMINATION. 

develop  some  unexpected  limitation,  it  must  be  looked 
upon  as  a  competitor  of  the  incandescent  that,  although 
not  now  serious,  may  become  so  at  any  time,  and  per- 
haps to  a  very  material  extent. 

Following  up  the  question  of  higher  luminous  ef- 
ficiency than  that  given  by  the  incandescent  lamp,  one  nat- 
urally turns  to  the  vacuum  tube,  in  which  the  illuminant 
is  not  a  heated  solid,  but  an  incandescent  gas.  It  has 
long  been  well  known  that  an  electric  discharge  passed 
through  a  tube  of  highly  rarefied  gas  causes  the  tube 
to  become  the  seat  of  very  brilliant  luminous  phenom- 
ena. The  light  produced,  however,  does  not  form  a  con- 
tinuous spectrum,  as  does  an  incandescent  solid,  but 
gives  a  spectrum  of  bright  bands  or  lines.  This  fact  of 
itself  gives  some  hope  of  efficiency,  for  it  is  the  plentiful 
production  of  useless  wave  lengths  that  renders  an  ordi- 
nary incandescent  body  so  inefficient  a  source  of  light. 
If  a  gas  could  be  found  giving  a  brilliant  spectrum  of 
bands,  all  of  useful  wave  lengths,  one  might  expect  that 
a  considerable  proportion  of  the  energy  applied  to  the 
tube  would  turn  up  as  useful  illumination.  Or  if  not 
a  single  gas,  then  a  combination  of  gases  might  be  found 
such  as  would  answer  the  purpose.  During  the  past  ten 
years  much  work  has  been  put  in  along  this  line  by  Mr. 
Tesla  and  others,  but  as  yet  without  the  production  of  a 
commercial  lamp,  although  very  magnificent  effects 
have  been  produced  experimentally. 

The  difficulties  which  have  been  met  are,  first,  the  need 
under  ordinary  conditions  of  rather  high  voltage  in  the 
discharge,  the  difficulty  of  obtaining  a  steady  light  of 
good  color,  and,  most  of  all,  the  production  of  anything 
like  a  practicable  intrinsic  brilliancy.  The  brightness  of 
an  ordinary  vacuum  tube  is  apt  to  be  greatly  overrated, 


THE  ELECTRIC  INCANDESCENT  LAMP.     135 

seen  as  it  usually  is  with  the  room  otherwise  in  darkness, 
and  a  tallow  candle  will  make  a  pretty  bright  vacuum 
tube  look  pale  as  a  wisp  of  fog. 

Now,  low  intrinsic  brilliancy  is  not  in  itself  at  all  ob- 
jectionable, but  in  the  matter  under  discussion  it  con- 
notes a  radiant  of  large  dimensions.  This  means  that 
either  there  must  be  an  enormous  multiplicity  of  small 
tubes  or  else  a  few  tubes  so  large  as  to  involve  rather 
high  electromotive  force  in  driving  the  discharge 
through  them.  In  large  tubes  the  light  is  generally  very 
unsteady,  and  the  best  effects  seem  to  be  gotten  from 
long  coiled  tubes  of  small  diameter,  which  are  not  easy 
to  excite.  As  to  color,  there  is  a  strong  tendency  toward 
a  bluish  or  greenish  hue,  which  will  have  to  be  removed 
before  a  practical  lamp  is  produced.  It  is  easy  to  find 
gaseous  mixtures  free  from  this  objection,  but  perhaps  at 
a  considerable  loss  of  efficiency  or  other  disadvantage. 
The  otherwise  promising  mercury  vapor  lamp  is  ex- 
tremely bad  in  color. 

The  efficiency  of  the  light  produced  by  vacuum  tubes 
has  been  several  times  measured.  It  appears  that  the 
luminous  efficiency,  that  is,  the  proportion  of  radiant 
energy  from  the  tube  which  is  of  luminous  wave  lengths, 
is  something  like  25  or  30  per  cent.,  five  or  six  times 
better  than  in  case  of  the  incandescent  lamp. 

If  the  tubes  are  forced  to  anything  like  the  intrinsic 
brilliancy  of  even  the  dullest  flames,  secondary  phe- 
nomena involving  heating  seem  to  arise,  considerably 
decreasing  the  efficiency,  so  that  it  seems  to  be  still  a 
long  step  from  our  present  vacuum  tubes  to  "  light  with- 
out heat."  This  "  light  without  heat  "  implies  radiant 
energy  that  is  nearly  or  quite  all  luminous.  But  this 
condition  might  be  fulfilled  and  the  light  yet  be  most 


136  THE   ART   OF   ILLUMINATION. 

impractical,  as,  for  example,  in  a  sodium  flame,  which 
gives  an  effect  altogether  ghastly.  Any  monochromatic 
light  is  utterly  destructive  of  color,  but  it  might  be  possi- 
ble so  to  combine  nearly  monochromatic  lights  of  the 
primary  red,  blue,  and  green  as  to  obviate  this  difficulty. 
Or,  it  may  be  eventually  possible  to  excite  luminous 
radiation  in  gases  or  even  in  solids  so  as  to  get  results 
quite  different  from  the  ordinary  spectra  of  the  bodies. 
The  vacuum  tube  lamp  is  probably  capable  of  develop- 
ment into  a  commercial  method  of  illumination  for  some 
purposes,  but  in  any  form  in  which  it  has  yet  been  sug- 
gested it  must  be  regarded  rather  as  a  stepping-stone 
on  the  way  toward  light  without  heat  than  as  the  thing 
itself.  It  is  given  this  place  in  a  discussion  of  practical 
illuminants,  not  on  account  of  its  present  position,  but 
because  the  author  is  very  strongly  of  the  opinion  that  it 
may  advance  to  some  degree  of  importance  at  almost 
any  moment,  and  because  it  gives  promise  of  an  efficiency 
considerably  beyond  anything  which  has  hitherto  been 
reached  in  artificial  illuminants. 

But  it  may  be  that  we  must  look  to  the  chemist  rather 
than  the  electrician  for  the  final  word  as  to  i. /animation. 
It  is  well  within  the  bounds  of  possibility  that  some  ex- 
aggerated prosphorescence  may  be  found  which  will 
enable  us  to  store  solar  energy  directly  for  use  at  night. 
Or  the  firefly  may  give  up  his  secret  and  teach  us  how 
to  get  light  by  chemical  changes  at  low  temperatures. 
And  the  firefly  knows.  The  light  emitted  by  such  light- 
giving  insects  is  unique  and  most  extraordinary  in  its 
properties.  For  so  far  as  can  be  ascertained  the  total 
radiant  energy  lies  within  the  limits  of  the  visible 
spectrum,  and  not  only  there,  but  in  the  most  brilliant 
part  thereof.  No  similar  distribution  of  radiant  energy 


THE  ELECTRIC   INCANDESCENT  LAMP.     137 

is  elsewhere  known.  The  ordinary  firefly  of  this  coun- 
try is  typical  of  the  whole  class,  giving  a  greenish-white 
light  that,  when  examined  in  the  spectroscope,  shows 
a  brilliant  band  extending  over  the  yellow  and  green  and 
fading  rapidly  as  the  red  and  blue  are  approached. 

Professor  S.  P.  Langley  has  carefully  investigated  the 
radiation  from  Pyrophorus  noctilucus,  a  West  Indian 
species  which  attains  a  length  of  i  1-2  inch,  and  of  which 
a  half  dozen  specimens  in  a  bottle  give  sufficient  light 
for  reading.  These  insects  gave  spectra  bright  enough 
to  permit  careful  investigation,  and  by  comparison  with 
solar  light  reduced  to  the  same  intensity,  Langley  found 
the  curves  shown  in  Fig.  45.  Here  B  is  the  light  radi- 
ated by  the  insect  and  A  solar  light.  The  curves  show 
by  the  ordinates  at  each  point  the  relative  intensities  of 
the  various  parts  of  the  spectra. 

It  at  once  appears  that  the  light  of  Pyrophorus  in- 
cludes much  the  less  range  of  color,  and  is  much  the 
richer  in  yellow  and  green.  The  maximum  intensity 
is  very  near  the  beginning  of  the  clear  green  (at  about 
wave  length  5500),  and  the  light  extends  only  a  little 
way  into  the  red  and  the  blue.  Fig.  45,  which  shows  the 
apparent  distribution  of  light  indicated  by  the  two 
curves,  exhibits  the  narrow  limits  of  the  radiation  from 
Pyrophorus  very  clearly.  The  spectrum  is  practically 
limited  by  the  solar  lines  C  and  F,  and  Langley's  most 
careful  experiments  showed  nothing  perceptible  outside 
of  these  limits,  a  most  remarkable  state  of  affairs,  quite 
standing  alone  in  our  knowledge  of  radiation. 

For  equal  light  Langley  found  that  Pyrophorus  ex- 
pends only  about  1-400  of  the  energy  required  by  a 
candle  or  gas  flame.  This  fact  gives  us  a  clew  to  the 
efficiency  of  Pyrophorus  as  a  light  producer.  It  appears 


i38 


THE   ART   OF   ILLUMINATION. 


to  be  about  five  candles  per  watt,  perhaps  even  a  little 
better — fifteen  or  twenty  times  the  efficiency  of  an  incan- 


PHOTOMETRIO    CURVES. 
FOR  EQUAL  TOTAL  AMOUNTS  OF  LIGHT. 

A — Sun-light. 
B- Fire-fly  light. 

ABSCISSAE.— WAVE  LENGTHS. 
ORDINATES.— LUMINOUS  INTENSITIES. 


Fig.  45.— Curves  of  Firefly  Light  and  Solar  Light. 

descent  lamp,  about  four  times  the  efficiency  of  an  arc  lamp 
at  its  best.     It  is  a  startling  lesson.     The  light-giving 


THE    ELECTRIC   INCANDESCENT   LAMP.    139 

process  of  Pyrophonts  is  apparently  the  slow  oxidation 
of  some  substance  produced  by  it.  Even  if  this  sub- 
stance could  be  reproduced  in  the  laboratory  the  light 
would  be  too  nearly  monochromatic  to  be  satisfactory  as 
an  illuminant,  but  it  presumably  is  within  the  range  of 
possibility  to  obtain  a  combination  of  phosphorescent 
substances  which  would  give  light  of  better  color  at  very 
high  efficiency. 

Certainly  the  problem  is  a  most  fascinating  one,  and 
whether  the  ultimate  solution  lies  in  vacuum-tube  light- 
ing or  in  some  form  of  phosphorescence,  one  may  say 
with  an  approach  to  certainty  that  all  forms  of  incandes- 
cence of  highly  heated  solids  are  too  inefficient  in  giving 
light  to  approach  the  economy  desirable  in  an  artificial 
illuminant.  Indeed,  a  solid  substance  of  great  light- 
giving  efficiency,  when  heated  to  incandescence,  would 
be  somewhat  of  an  anomaly,  since  it  would  probably  have 
to  possess  an  enormous  specific  heat  at  moderate  tempera- 
tures. What  is  really  needed  is  some  method,  chemical 
or  electrical,  of  passing  by  the  slow  vibrations  that  char- 
acterize radiant  heat  and  stirring  up  directly  vibrations 
of  the  frequency  corresponding  to  light.  The  vacuum 
tube  gives  the  nearest  approach  to  a  solution  of  this 
problem  yet  devised,  but  it  still  leaves  much  to  be  de- 
sired, and  there  is  plenty  of  work  for  the  investigator. 


CHAPTER    VII. 

THE    ELECTRIC    ARC    LAMP. 

THE  electric  arc  is  the  most  intense  artificial  illuminant 
and  the  chief  commercial  source  of  very  powerful  light. 
A  full  account  of  it  would  make  a  treatise  by  itself,  so 
that  we  can  here  treat  only  the  phases  of  the  subject 
which  bear  directly  on  its  place  as  a  practical  illuminant. 
First  observed  probably  by  Volta  himself,  the  arc  was 
brought  to  general  notice  by  Davy  in  1808  in  the  course 
of  his  experiments  with  the  great  battery  of  the  Royal 
Institution.  If  one  slowly  breaks  at  any  point  an -elec- 
tric circuit  carrying  considerable  current  at  a  fair  voltage 
the  current  does  not  cease  flowing  when  the  conductor 
becomes  discontinuous,  but  current  follows  across  the 
break  with  the  evolution  of  great  heat  and  a  vivid  light. 
If  the  separation  is  at  the  terminals  of  two  carbon  rods 
the  light  is  enormously  brilliant,  and  by  proper  mechan- 
ism can  be  maintained  tolerably  constant.  The  passage 
of  the  current  is  accompanied  by  the  production  of  im- 
mense heat,  and  the  tips  of  the  carbon  rods  grow  white 
hot,  and  serve  as  the  source  of  light.  In  an  ordinary 
arc  lamp  the  upper  carbon  is  the  positive  pole  of  the 
circuit,  and  is  fed  slowly  downward,  so  as  to  keep  the 
arc  uniform  as  the  carbon  is  consumed.  The  main  con- 
sumption of  energy  appears  to  be  at  the  tip  of  this  posi- 
tive carbon,  which  is  by  far  the  most  brilliant  part  of 
the  arc,  and  at  which  the  carbon  fairly  boils  away  into 


THE   ELECTRIC   ARC   LAMP.  141 


Fig.  46.— The  Electric  Arc. 

vapor,  producing  a  slight  hollow  in  the  center  of  the 
upper  carbon,  known  as  the  "  crater." 

The  carbon  outside  the  crater  takes  the  shape  of  a 
blunt  point,  while  the  lower  carbon  is  rather  evenly  and 
more  sharply  pointed,  and  tends,  if  the  arc  is  short, 
to  build  up  accretions  of  carbon  into  somewhat  of  a 


142 


THE   ART   OF   ILLUMINATION. 


mushroom  shape.  Fig.  46  shows  the  shape  of  these  tips 
much  enlarged,  as  they  would  appear  in  looking  at  the 
arc  through  a  very  dark  glass.  Under  such  circum- 
stances the  light  from  the  arc  between  the  carbon  points 
seems  quite  insignificant,  and  it  is  readily  seen  that  the 
crater  is  by  far  the  hottest  and  most  brilliant  region.  In 

80 


GO 


•540 

c 


0  200  400  COO 

Mean  Hemispherical  c.p.   Lower  Hemisphere 

Fig.  47. — Relation  between  Current  Density  and  Intensity. 

point  of  fact  the  crater  is  at  a  temperature  of  probably 
3500  to  4000  degrees  F.,  and  gives  about  50,000  candle- 
power  per  square  inch  of  surface — -sometimes  even  more. 
It  is  obvious  that  the  more  energy  spent  in  this  crater 
the  more  heat  and  light  will  be  evolved,  and  that  the 
concentration  of  much  energy  in  a  small  crater  ought  to 
produce  a  tremendously  powerful  arc.  It  is  not  surpris- 
ing therefore  to  find  that  the  larger  the  current  crowded 
through  a  small  carbon  tip, — in  other  words,  the  higher 
the  current  density  in  the  arc, — the  more  intense  the 
luminous  effects  and  the  more  efficient  the  arc.  Fig.  47 


THE   ELECTRIC   ARC   LAMP.  143 

shows  this  fact  graphically,  giving  the  relation  between 
current  density  and  light  in  an  arc  maintained  at  uni- 
form current  and  voltage. 

The  change  in  density  of  current  was  obtained  by 
varying  the  diameter  of  the  carbons  employed,  the 
smallest  being  about  5-16  inch  in  diameter,  the  largest 
3-4  inch.  The  current  was  6.29  amperes,  and  the  voltage 
about  43.5.  The  efficiency  of  the  arc  appears  from  these 
experiments  to  be  almost  directly  proportional  to  the 
current  density.  But  if  the  carbon  is  too  small  it  wastes 
away  with  inconvenient  rapidity,  while  if  it  be  too  large 
the  arc  does  not  hold  its  place  steadily  and  the  carbon 
gets  in  the  way  of  the  light. 

The  higher  the  voltage  the  longer  arc  can  be  success- 
fully worked,  but  here  again  there  are  serious  limita- 
tions. With  too  short  an  arc  the  carbons  are  in  the  way 
of  the  light,  and  the  lower  carbon  tends  to  build  up 
mushroom  growths,  which  interfere  with  the  formation 
of  a  proper  arc.  In  arcs  worked  in  the  open  air  the  arc 
is  ordinarily  about  3-32  inch  long.  If  the  voltage  is 
raised  above  the  40  to  45  volts  at  the  arc  commonly 
employed  for  open  arcs,  the  crater  temperature  seems 
to  fall  off  and  the  arc  gets  bluish  in  color  from  the  rela- 
tively larger  proportion  of  light  radiated  by  the  glow- 
ing vapor  between  the  carbon  poles. 

So  it  comes  about  that  commercial  arcs  worked  in  the 
open  air  generally  run  at  from  45  to  50  volts,  and  from 
6  to  10  amperes.  The  softer  and  finer  the  carbons  the 
lower  the  voltage  required  to  maintain  an  arc  of  good 
efficiency  and  proper  length,  so  that  arcs  can  be  worked 
successfully  at  25  to  35  volts  with  proper  carbons,  and 
with  very  high  efficiency,  but  at  the  cost  of  burning  up 
the  carbons  rather  too  rapidly.  Abroad,  where  both 


144  THE   ART   OF   ILLUMINATION. 

high-grade  carbons  and  labor  are  cheaper  than 
in  this  country,  such  low  voltage  arcs  are  freely  used 
with  excellent  results,  and  give  a  greatly  increased  effi- 
ciency. 

Sometimes  three  are  burned  in  series  across  no-volt 
mains,  where  in  American  practice  one,  or  at  most  two 
arc  lamps,  would  be  used  in  series  with  a  resistance  coil, 
the  same  amount  of  energy  being  used  in  each  case. 
With  proper  carbons  too,  a  steady  and  efficient  arc  can 
be  produced  taking  only  3  or  4  amperes,  and  admirable 
little  arc  lamps  of  such  kind  are  in  use  on  the  Continent. 
The  carbons  are  barely  as  large  as  a  lead  pencil  and  the 
whole  lamp  is  proportionately  small,  but  the  light  is 
brilliant  and  uniform. 

The  upper  carbon  burns  away  about  twice  as  fast  as 
the  lower,  and  the  rate  of  consumption  is  ordinarily 
from  i  to  2  inches  per  hour,  according  to  the  diameter 
and  hardness  of  the  carbons. 

The  carbons  themselves  are  generally  about  1-2  inch 
in  diameter,  and  one  or  both  are  often  cored,  i.  e.,  pro- 
vided with  a  central  core,  perhaps  1-16  inch  in  diameter, 
of  carbon  considerably  softer  than  the  rest.  This  tends 
to  hold  the  arc  centrally  between  the  carbons  and  also 
steadies  it  by  the  greater  mass  of  carbon  vapor  provided 
by  the  softer  portion.  Generally  it  is  found  sufficient 
to  use  one  cored  and  one  solid  carbon  in  each  arc,  al- 
though in  this  country  arcs  burning  in  the  open  air  usu- 
ally are  provided  with  solid  carbons  only. 

In  American  practice  such  open  arcs  are  very  rapidly 
passing  out  of  use,  and  are  being  replaced  by  the  so- 
called  enclosed  arcs.  During  the  past  three  or  four 
years  these  have  gone  into  use  in  immense  numbers, 
until  at  the  present  time  the  open  arc  is  very  rarely  in- 


THE   ELECTRIC   ARC   LAMP.  145 

stalled,  and  illuminating  companies  are  discarding 
them  as  rapidly  as  they  find  it  convenient  to  purchase 
equipment  for  the  enclosed  arcs. 

The  principle  of  the  enclosed  arc  is  very  simple.  It 
merely  consists  in  fitting  around  the  lower  carbon  a  thin 
elongated  vessel  of  refractory  glass  with  a  snugly  fitting 
metallic  cap  through  which  the  upper  carbon  is  fed,  the 
fit  being  as  close  as  permits  of  proper  feeding.  The 
result  is  that  the  oxygen  is  rapidly  burned  out  of  the 
globe,  and  the  rapid  oxidation  of  the  carbon  ceases,  the 
heated  gas  within  checking  all  access  of  fresh  air  save 
for  the  small  amount  that  works  in  by  diffusion  through 
the  crevices. 

The  carbon  wastes  away  at  the  rate  of  only  something 
like  1-8  inch  per  hour  under  favorable  circumstances, 
and  the  lamp,  only  requires  trimming  once  in  six  or 
eight  full  nights  of  burning,  instead  of  each  night.  For 
all-night  lighting  it  used  to  be  necessary  to  employ  a 
double  carbon  lamp,  in  which  were  placed  two  pairs  of 
carbons,  so  that  when  the  first  pair  was  consumed  the 
second  pair  would  automatically  go  into  action  and  fin- 
ish out  the  night.  The  enclosed  lamp  burns  100  hours 
or  more  with  a  single  trimming.  Even  much  longer 
burning  than  this  has  been  obtained  from  a  1 2-inch 
carbon,  such  as  is  customarily  used,  but  one  cannot 
safely  reckon  on  a  better  performance  without  very  un- 
usual care. 

Fig.  48  shows  a  typical  enclosed  arc  lamp,  of  the  de- 
scription often  used  on  no-volt  circuits,  both  with  and 
without  its  outer  globe  and  case.  The  nature  of  the 
inner  globe  is  at  once  apparent,  but  it  should  also  be 
noted  that  the  clutch  by  which  the  carbon  is  fed  acts, 
as  in  many  recent  lamps,  directly  upon  the  carbon  itself, 


146 


THE   ART   OF   ILLUMINATION. 


thereby  saving  the  extra  length  of  lamp  required  by  the 
use  of  a  feeding  rod  attached  to  the  carbon.  Finally, 
at  the  top  of  the  lamp  is  seen  a  coil  of  spirally  wound 
resistance  wire.  The  purpose  of  this  is  to  take  up  the 


Fig.  48. — Typical  Enclosed  Arc  Lamp. 

difference  between  no  volts,  which  is  the  pressure  at 
the  mains,  and  that  voltage  which  it  is  desired  to  use  at 
the  arc  and  in  the  lamp  mechanism. 

Such  a  resistance  evidently  involves  a  considerable 
waste  of  energy,  but  in  the  enclosed  arc  the  voltage 
at  the  arc  itself  is,  of  necessity,  rather  high,  70  to  75 
volts,  so  that  the  waste  is  less  than  it  would  otherwise  be. 

It  has  been  found  that  when  burning  in  an  inner  globe 


THE   ELECTRIC   ARC   LAMP.  147 

without  access  of  air,  the  lower  or  negative  carbon  be- 
gins to  act  badly,  and  to  build  up  a  mushroom  tip,  when 
the  voltage  falls  below  about  65  volts.  Hence  it  is  neces- 
sary to  the  successful  working  of  the  scheme  that  the 
arc  should  be  nearly  twice  as  long  as  when  the  carbons 
are  burning  in  open  air.  This  has  a  double  effect,  in 
part  beneficial,  in  part  harmful.  With  the  increased 
length  the  crater  practically  disappears,  and  the  light  is 
radiated  very  freely  without  being  blocked  by  the  car- 
bons. Hence  the  distribution  of  light  from  the  enclosed 
arc  is  much  better  than  from  an  open  arc. 

On  the  other  hand,  there  is  no  point  of  the  carbon  at 
anything  like  the  temperature  of  the  typical  open  arc 
"  crater,"  and  the  intrinsic  efficiency  is  thereby  lowered. 
Also  if  the  enclosed  arc  is  to  take  the  same  energy  as  a 
given  open  arc,  the  current  in  the  former  must  be  re- 
duced in  proportion  to  the  increased  voltage,  hence, 
other  things  being  equal,  the  current  density  is  lowered, 
wrhich  also  lowers  the  efficiency. 

The  compensation  is  found  in  the  lessened  care  and 
the  lessened  annual  cost  for  carbons.  The  carbons  them- 
selves have  to  be  of  a  special  grade,  and  are  about  two 
and  a  half  times  as  expensive  as  plain  solid  carbons,  but 
the  number  used  is  so  small  that  the  total  cost  is  low. 
There  is  some  extra  expense  on  account  of  breakage  of 
the  inner  globes,  but  the  saving  in  labor  and  carbons 
far  outweighs  this.  Moreover,  the  light,  albeit  some- 
what bluish  white,  is  much  steadier  than  that  of  the 
ordinary  open  arc,  and  the  inner  globe  has  material 
value  in  diffusing  the  light,  being  very  often  of  opal 
glass,  so  that  the  general  effect  is  much  less  dazzling 
than  that  of  an  open  arc,  and  the  light  is  far  better  dis- 
tributed. 


148  THE   ART   OF  ILLUMINATION. 

In  outdoor  lighting  the  greater  proportion  of  hori- 
zontal rays  from  the  enclosed  arc  is  of  considerable  bene- 
fit, while  in  buildings  the  same  property  increases  the 
useful  diffusion  of  light,  as  will  be  presently  shown. 
Of  course,  when  enclosed  arcs  are  operated  in  series,  as 
in  street  lighting,  the  resistance  of  Fig.  48  is  reduced 
to  a  trivial  amount,  or  abolished,  so  that  the  extra  voltage 
required  with  the  enclosed  arc  is  the  only  thing  to  be 
considered.  The  enclosed  arc  used  in  this  way  is  very 
materially  better  as  an  illuminant  than  an  open  arc  tak- 
ing the  same  current,  and  experience  shows  that  it  may 
be  substituted  for  an  open  arc,  taking  about  the  same 
energy,  with  general  improvement  to  the  illumination. 

The  weak  point  of  the  open  arc  is  its  very  bad  distri- 
bution of  light,  which  hinders  its  proper  utilization.  The 
fact  that  most  of  the  light  is  delivered  from  the  crater 
in  the  upper  carbon  tends  to  throw  the  light  downward 
rather  than  outward,  and  much  of  it  is  intercepted  by  the 
lower  carbon.  Fig.  49  gives  from  Wybauw's  experi- 
ments the  average  distribution  of  light  from  26  different 
arc  lamps,  representing  the  principal  American  and 
European  manufacturers.  The  radii  of  the  curve  give 
the  intensities  of  the  light  in  various  angles  in  a  vertical 
plane.  The  distribution  of  light  in  space  would  be  nearly 
represented  by  revolving  this  curve  about  a  vertical  axis 
passing  through  its  origin,  although  at  any  particular 
moment  the  distribution  of  light  from  an  arc  may  be  far 
from  equal  on  the  two  sides. 

The  shape  of  the  curve  is  approximately  a  long  ellipse 
with  its  major  axis  inclined  40  degrees  below  the  hori- 
zontal. The  presence  of  globes  on  the  lamps  may  mod- 
ify this  curve  somewhat,  but  in  ordinary  open  arcs  it 
always  preserves  the  general  form  shown.  The  small 


THE   ELECTRIC   ARC   LAMP. 


149 


portion  of  the  curve  above  the  horizontal  plane  shows 
the  light  derived  from  the  lower  carbon  and  the  arc 
itself,  while  the  major  axis  of  the  curve  measures  the 
light  derived  from  the  crater.  The  tendency,  then,  of  the 
open  arc  is  to  throw  a  ring  of  brilliant  light  downward 


80°  60° 

Fig.  49. — Distribution  of  Light  from  an  Open  Arc. 

at  an  angle  of  40  degrees  below  the  horizontal,  so  that 
within  that  ring  the  light  is  comparatively  weak,  and 
without  it  there  is  also  considerable  deficiency.  Hence 
the  open  arc,  if  used  out  of  doors,  fails  to  throw  a  strong 
light  out  along  the  street,  while  the  illumination  is  daz- 
zling in  a  zone  near  the  lamp. 

For  the  same  reason  the  open  arc  is  at  a  disadvantage 
in  interior  lighting,  for  the  reason  that  most  of  the  light, 
being  thrown  downward,  falls  upon  things  and  surfaces 
far  less  effective  for  diffusion  than  the  ordinary  walls 
and  ceiling.  Hence  one  of  the  very  best  ways  of  using 
arcs  for  interior  lighting  is  to  make  the  lower  carbon 
positive  instead  of  the  upper,  and  to  cut  off  all  the  down- 
ward light  by  a  reflector  placed  under  the  lamp.  Then 


ISQ  THE   ART    OF   ILLUMINATION. 

practically  all  the  light  is  sent  upward  and  outward  to 
be  diffused  by  the  walls  and  ceiling. 

The  enclosed  arc,  on  the  other  hand,  gives  a  much 
rounder,  fuller  curve  of  distribution,  the  light  being 
thrown  well  out  toward  the  horizontal  and  there  being 
a  pretty  strong  illumination  above  the  horizontal.  For 
the  same  energy  the  maximum  illumination  is  little  more 
than  half  the  maximum  derived  from  an  open  arc,  but 
the  result  in  distribution  is  such  «»  10  fully  compensate 
for  this  difference  if  one  considers  the  lamps  as  illumi- 
nants  and  not  merely  as  devices  for  transforming  electri- 
cal into  luminous  energy. 

Fig.  50  shows  a  composite  distribution  curve  from  ten 
or  a  dozen  enclosed  arc  lamps,  such  as  are  used  on  con- 
stant potential  circuits,  including  various  makes.  Most 
of  them  were  lamps  taking  about  5  amperes,  and  there- 
fore using  nearly  400  watts  at  the  arc,  besides  the  energy 
taken  up  in  the  resistance  and  the  mechanism.  Figs.  49 
and  50  afford  a  striking  contrast  in  distribution,  and  it  is 
at  once  obvious  that  the  lamps  represented  by  the  latter 
have  a  great  advantage  as  general  illuminants  either  in- 
doors or  outside.  These  figures  include  the  inner  globe, 
of  course,  generally  of  opal  glass,  which  is  of  some  bene- 
fit in  correcting  the  bluish  tinge  which  is  produced  by 
the  long  arc.  After  a  few  hours'  burning  a  slight  film 
collects  on  the  inner  globe,  which  tends  to  the  same  re- 
sult. For  interior  lighting,  outer  globes  of  opal  or 
ground  glass  are  generally  added,  so  that  the  color 
question  is  partially  eliminated. 

As  ordinarily  employed,  enclosed  arc  lamps  take  from 
5  to  7  amperes,  although  now  and  then  3  or  4  ampere 
lamps  are  used.  These  smaller  sizes  are  less  satisfac- 
tory in  the  matter  of  color  of  the  light,  and  are  not  widely 


THE   ELECTRIC   ARC   LAMP. 


used.  Abroad  open  arcs  taking  as  little  as  2.5  amperes 
are  sometimes  used.  The  carbons  in  this  case  are  very 
slender  and  of  particularly  fine  quality,  and  these  tiny 
lamps  can  be  made  to  give  an  admirable  light. 

Outside  of  America,  the  enclosed  arc  is  little  used,  for 
abroad  labor  is  much  cheaper  than  here,  and  carbons  of 


60*      50"        4'J 


Fig.  50. — Distribution  of  Light  from  Enclosed  Arc. 

a  grade  costly  or  quite  unattainable  here  are  there  rea- 
sonably cheap,  so  that  the  somewhat  higher  efficiency 
of  the  open  arc  compensates  for  the  extra  labor  and 
carbons.  Aside  from  this  the  bluish  tinge  of  the  light 
from  enclosed  arcs  of  small  amperage  is  considered  ob- 
jectionable, and  the  gain  in  steadiness  so  conspicuous  in 
American  practice  almost  or  quite  disappears  when  the 
comparison  is  made  with  open  arcs  taking  the  carbons 
available  abroad. 

At  its  best  the  electric  arc  has  fully  three  times  the 
efficiency  of  a  first-class  incandescent  lamp,  but  this  ad- 
vantage is  somewhat  reduced  by  the  need  of  diffusing 
globes  to  keep  down  the  dazzling  effect  of  the  arc,  and 


152  THE   ART   OF   ILLUMINATION. 

to  correct  the  distribution  of  the  light.  Taking  these 
into  account,  and  also  reckoning  the  energy  wasted  in 
the  resistances  in  case  of  arc  lamps  worked  from  con- 
stant potential  circuits,  the  gain  in  efficiency  is  consid- 
erably reduced,  and  if  one  also  figures  the  better  illumi- 
nation obtained  by  using  distributed  lights  in  incandes- 
cent lighting,  the  arc  lamp  has  a  smaller  advantage  than 
is  generally  supposed.  Many  experiments  bearing  on 
this  matter  have  been  made,  and  a  study  of  the  results  is 
highly  instructive. 

By  far  the  most  complete  investigation  of  the  proper- 
ties of  the  enclosed  type  of  arc  lamps  is  that  recently  made 
by  a  committee  of  the  National  Electric  Light  Associa- 
tion. The  investigation  was  upon  the  arc  lamps  both  for 
direct  and  alternating  currents,  as  customarily  used  on 
constant-potential  circuits.  The  results,  however,  are  not 
materially  different,  so  far  as  distribution  of  light  goes, 
from  those  that  belong  to  similar  lamps  for  series  circuits. 
Fig.  50  is  the  composite  curve  of  distribution  obtained 
by  this  committee  in  the  tests  of  direct-current  lamps. 

The  individual  curves  vary  somewhat,  although  show- 
ing the  same  essential  characteristics.  Fig.  51  shows  a 
typical  example  both  with  the  outer  globe  opalescent,  like 
the  inner  globe,  and  also  with  a  clear  outer  globe.  The 
effect  of  the  former  in  reducing  and  also  in  diffusing  the 
light  is  very  conspicuous.  The  opalescent  globe  absorbed 
a  little  over  14  per  cent,  of  the  light.  This  absorption  is 
much  less  than  would  be  given  by  a  ground  or  milky  glass 
shade,  but  it  serves  to  cut  down  the  intrinsic  brilliancy  to 
a  useful  degree.  A  clear  globe  absorbs  about  10  per  cent. 

The  weak  point  of  such  lamps  as  efficient  illuminants 
lies  in  the  large  amount  of  energy  wasted  in  the  lamp 
mechanism,  including  the  resistance  for  reducing  the 


THE   ELECTRIC   ARC   LAMP. 


J53 


voltage  of  the  mains  to  that  desirable  for  the  enclosed  arc. 
This  loss  amounts  ordinarily  to  nearly  30  per  cent,  of  the 
total  energy  supplied,  so  that  while  the  arc  itself  is  highly 
efficient,  the  lamps  as  used  are  wasteful.  No  one  but  an 
American  would  think  of  working  a  75-volt  arc  off  a  120- 


Fig.  51. — Effect  of  Globes  on  Enclosed  Arc. 

volt  circuit  and  absorbing  the  difference  in  an  energy- 
wasting  resistance,  but  the  advantages  of  the  enclosed  arc 
are  so  great  in  point  of  steadiness  and  moderate  cost  of 
labor  that  the  practice  has  been  found  commercially  ad- 
vantageous, and  the  open  arc  has  been  practically  driven 
from  the  field  for  all  indoor  illumination,  and  is  being 
rapidly  displaced  in  street  lighting. 


154  THE   ART    OF   ILLUMINATION. 

Foreign  practice  tends,  as  already  noted,  toward  the  use 
of  two  or  even  three  open  arcs  in  series  on  constant  poten- 
tial circuits.  These  can  be  fitted  with  diffusing  globes  to 
keep  the  intrinsic  brilliancy  within  bounds,  and  obviously 
give  a  far  larger  amount  of  light  for  the  energy  consumed 
than  is  obtained  here  with  enclosed  arcs,  but  we  have 
neither  cheap  high-grade  carbons  nor  cheap  labor,  and  as 
in  the  last  resort  the  thing  which  determines  current  prac- 
tice must  be  the  total  cost  of  light  per  candle-hour,  it  is 
likely  that  both  methods  of  lighting  are  right  when  judged 
by  their  respective  conditions. 

At  present  alternating-current  arc  lights  are  being 
rather  widely  used,  both  on  constant  potential  and  on 
constant-current  circuits,  and  such  arcs  present  some  very 
interesting  characteristics.  Evidently  when  an  arc  is 
formed  with  an  alternating  current  there  is  no  "  positive  " 
and  no  "  negative  "  carbon,  each  carbon  being  positive 
and  negative  alternately,  and  changing  from  one  to  the 
other  about  7200  times  per  minute — 120  times  per 
second. 

Under  these  circumstances  no  marked  crater  is  formed 
on  either  carbon,  and  the  two  carbons  are  consumed  at 
about  an  equal  rate.  As  a  natural  result  of  the  intermit- 
tent supply  of  energy  and  the  lack  of  a  localized  crater, 
the  average  carbon  temperature  is  somewhat  lower  than 
in  case  of  the  direct-current  arc,  and  the  real  efficiency  of 
the  arc  as  an  illuminant  is  also  somewhat  lowered. 
Tests  made  to  determine  this  difference  of  efficiency  have 
given  somewhat  varied  results,  but  it  seems  probable  that 
for  unit  energy  actually  applied  to  the  arc  itself  the  direct- 
current  arc  will  give  somewhere  about  25  per  cent,  more 
light  than  the  alternating-current  arc.  But  since  when 
working  the  latter  on  a  constant  potential  circuit  the 


THE   ELECTRIC   ARC   LAMP.  155 

surplus  voltage  can  be  taken  up  in  a  reactive  coil,  which 
wastes  very  little  energy,  instead  of  by  a  dead  resistance, 
which  wastes  much,  the  two  classes  of  arcs  stand  upon  a 
more  even  footing  than  these  figures  indicate.  This  com- 
parison assumes  enclosed  arcs  in  each  case,  in  accordance 
with  present  practice. 

For  street  lighting,  as  we  shall  presently  see,  the  alter- 
nating arcs  have  certain  advantages  of  considerable  mo- 
ment with  respect  to  distribution,  so  that  as  practical 
illuminants  they  are  often  preferred. 

The  chief  objection  to  the  alternating-current  arc  has 
been  the  singing  noise  produced  by  it.  This  is  partly  due 
to  the  vibration  produced  in  the  lamp  mechanism  and 
partly  to  the  pulsations  impressed  directly  on  the  air  by 
the  oscillatory  action  in  the  arc  itself.  The  former  can  be 
in  great  measure  checked  by  proper  design  and  manu- 
facture, but  the  noise  due  directly  to  the  arc  is  much  more 
difficult  to  suppress. 

Abroad  where,  for  the  reason  already  adduced,  open 
arcs  are  commonly  used,  a  specially  fine,  soft  carbon  is 
used  for  the  alternating  arcs,  and  the  noise  is  hardly  per- 
ceptible. These  soft  volatile  carbons,  particularly  when 
used  at  a  considerable  current  density,  give  such  a  mass 
of  vapor  in  the  arc  as  to  endow  it  with  added  stability  and 
to  muffle  the  vibration  to  a  very  marked  degree.  The 
result  is  a  quiet,  steady,  brilliant  arc  of  most  excellent 
illuminating  power.  But  in  this  country  such  carbons 
are  with  difficulty  obtainable,  and,  even  if  they  were  to  be 
had  at  a  reasonable  price,  could  not  be  used  in  enclosed 
arc  lamps  on  account  of  rapid  smutting  of  the  inner  globe. 
Hence  it  is  by  no  means  easy  to  get  a  quiet  alternating 
arc,  and  in  a  quiet  interior  there  is  generally  a  very  per- 
ceptible singing,  pitched  about  a  semi-tone  below  bass  C, 


156  THE   ART    OF   ILLUMINATION. 

with  noticeable  harmonics,  a  kind  of  chorus  not  always 
desirable. 

In  selecting  alternating-current  lamps  for  indoor  work 
great  care  should  be  exercised  to  get  a  quiet  lamp.  Some 
of  the  American  lamps  when  fitted  with  tight  outer  globes 
and  worked  with  a  rather  large  current  are  entirely  unob- 
jectionable, but  in  many  cases  there  is  noise  in  the  mech- 
anism, or  the  globe  serves  as  a  resonator.  With  a  current 
of  7  to  7.5  amperes,  and  a  well  fitted  and  non-resonant 
globe,  little  trouble  is  likely  to  be  experienced.  Out  of 
doors,  of  course,  a  little  noise  does  not  matter. 

The  chief  characteristic  of  the  alternating  arc,  as  re- 
gards distribution  of  light,  is  its  tendency  to  throw  its 
light  outward  rather  than  downward  like  the  direct- 
current  arc;  in  fact,  considerable  light  is  thrown  above  the 
horizontal,  which  materially  aids  diffusion. 

For  this  reason  it  is  often  advantageous  to  use  reflect- 
ing shades  for  such  lamps,  so  as  to  throw  the  light  out 
nearly  horizontally  when  exterior  lighting  is  being  done. 
Indoors,  diffusion  answers  the  same  purpose,  unless 
powerful  downward  light  is  needed,  when  the  reflector  is 
of  service. 

Fig.  52,  from  the  committee  report  already  mentioned, 
shows  the  distribution  of  light  from  an  alternating- 
current  lamp,  with  an  opalescent  outer  globe,  with  a 
clear  outer  globe,  and  with  no  outer  globe,  and  with  a  por- 
celain reflecting  shade  of  the  form  indicated  by  the  dotted 
lines  in  the  figure.  The  abolition  of  the  outer  globe  and 
the  use  of  the  reflector  produces  a  prodigious  effect  in 
strengthening  the  illumination  in  the  lower  hemisphere, 
and  this  hemispherical  illumination  is  for  some  purposes 
a  convenient  way  of  reckoning  the  illumination  of  the 
lamp.  But  a  truer  test  is  the  spherical  candle-power, 


THE   ELECTRIC   ARC   LAMP.  157 

since  that  takes  account  of  all  the  light  delivered  by  the 
lamp.  Alternating  arc  lamps  seem  to  work  best  at  a  fre- 
quency of  50  to  60  cycles  per  second.  Above  60  cycles 
they  are  apt  to  become  noisy,  and  below  about  40  cycles 


Fig.  52. — Distribution  from  Alternating  Enclosed  Arc. 

the  light  flickers  to  a  troublesome  extent.  The  light  of 
the  alternating  arc  is  really  of  a  pulsatory  character,  owing 
to  the  alternations.  A  pencil  rapidly  moved  to  and  fro 
in  the  light  of  such  an  arc  shows  a  number  of  images — 
one  for  each  pulsation,  and  this  effect  would  be  very  dis- 
tressing if  one  had  to  view  moving  objects,  like  quick 


158 


THE   ART    OF   ILLUMINATION. 


running  machinery,  by  such  light.  A  harrowing  tale  is 
told  of  a  certain  theater  in  which  alternating  arcs  were 
installed  for  some  gorgeous  spectacular  effects,  and  of  the 
extraordinary  centipedal  results  when  the  ballet  -came  on. 

Ths  pulsation  is  somewhat  masked  when  the  enclosed 
arc  is  used,  even  with  a  clear  outer  globe,  and  is  generally 
rather  inconspicuous  when  an  opal  outer  globe  is  used. 
It  is  also  reduced  when  a  fairly  heavy  current  (7  to  8 
amperes)  is  used,  and  when  very  soft  carbons  are  em- 
ployed, as  they  can  be  in  open  arcs. 


WATTS 
CONSUMED 

MEAN   INTENSITY 
IN    H.    U. 

MEAN  WATTS 

I 

6 

1 

s 

O 

G 

jchanism 

Spherical 

Lower 
Hemi- 
spherical 

Spherical 
H.  U. 

Lower 
Hemi- 
spherical 

(H 

sS 

b 

*  4) 

hj 

Q 

O 

g 

£ 

£ 

°O 

OO 

Clear 

Os 

So 

Clear 

Outer 

Outer 

235 

332 

2-37 

1.66 

i 

5.01 

55i 

401 

150 

172 

256* 

362* 

3.10 

2.18* 

1.52* 

3 

5-08 

559 

406 

252 

i95 

216 

282 

2.85 

2.60 

1.99 

4 

4.76 

524 

381 

i43 

127 

139 

208 

4.12 

3-76 

2.52 

5 

4-i6t 

458 

333 

125 

154 

174 

221 

2.96 

2.63 

2.07 

7 

4-76 

524 

38i 

i43 

203 

333 

317 

2.63 

2.20 

1.65 

9 

4.84 

S32 

387 

MS 

182 

226 

28l 

2.83 

2.38 

1.89 

10 

4-99 

549 

399 

150 

202 

242 

309 

2-74 

2.24 

1.77 

12 

4.87 

-536 

390 

146 

I78 

195 

230 

3-°5 

2.66 

2-33 

Mean 

4.9 

529 

384 

M4 

I76 

207 

272 

3-03 

2.60 

1.98 

•  P. 

a 

^  u  a 

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«-.   !- 

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£  0  5 

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££3 

c 

O  ct  t-j 

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101 

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448 

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.82 

108 

127 

141 

206 

3-52 

3-i7 

2.17 

203 

236 

26 

1.94 

102 

6.79 

4S9 

.61 

37S 

•73 

84 

I46 

226t 

3.31 

.6ot 

I.72t 

103 

424 

.6s 

344 

•7rj 

80 

116 

130 

M7 

3.66 

•15 

2.88 

105 

6.20 

414 

.61 

,82 

.80 

.32 

128 

187 

219 

3-24 

.20 

1.89 

'53 

169 

•56 

2.23 

106 

6.12 

378 

•56 

208 

.70 

80 

132 

l82t 

284 

2.82 

.i9t 

i.48t 

108 

6.48 

4S7 

.64 

383 

.80 

74-5 

133 

175 

211 

3'3°* 

.61 

2.16 

no 
Mean 

6.18 
6.29 

339 
417 

•49 
.60 

276 
342 

.72 

.76 

63 
74-5 

140* 
130 

126 

143 
I90 

3-31 

2.68 
2.66 

2-37 
2.23 

*  Condition  of  no  outer  globe,     f  Condition  with  shade  on  lamp. 
NOTE. — All  marked  values  not  included  in  the  mean. 


THE   ELECTRIC   ARC   LAMP.  159 

An  interesting  comparison  of  direct-current  and  alter- 
nating-current enclosed  arcs,  as  used  on  constant  potential 
circuits,  is  found  in  the  foregoing  table,  from  the  report 
already  quoted. 

It  must  be  remembered  that  the  results  are  in  Hefner- 
Alteneck  units.  This  unit  is.  roughly  0.9  cp,  so  that  the 
mean  results,  reduced  to  a  candle-power  basis,  are,  for 
efficiency  when  using  clear  outer  globes,  as  follows : 

Direct-current  arc 2.89  watts  per  cp. 

Alternating  arc 2.96  watts  per  cp. 

These  efficiencies  are  on  their  face  but  little  better  than 
those  obtained  from  incandescent  lamps.  There  is  little 
doubt  that  as  a  matter  of  fact  a  given  amount  of  energy 
applied  to  3-watt  incandescent  lamps  will  give  more  use- 
ful illumination  than  if  used  in  arcs  of  the  types  here 
shown.  The  incandescents  lose  somewhat  in  efficiency, 
but  gain  by  the  fact  of  their  distribution  in  smaller  units. 

But  for  many  purposes  the  arcs  are  preferable  on  ac- 
count of  their  whiter  light  and  the  very  brilliant  illumi- 
nation that  is  obtainable  near  them. 

Both  direct  and  alternating-current  enclosed  arcs  gain 
by  the  use  of  rather  large  currents,  both  in  steadiness  and 
in  efficiency,  and  moreover  give  a  whiter  light.  The 
same  is  true,  for  that  matter,  of  open  arcs,  in  which  the 
larger  the  current  the  higher  the  efficiency.  Very  many 
experiments  on  the  efficiency  of  open  arcs  have  been  made 
with  moderately  concordant  results.  Their  efficiency 
ranges  in  direct-current  arcs  from  about  1.25  watts  per 
candle  in  the  smallest  to  about  0.6  or  a  little  less  in  the 
most  powerful.  Fig.  53  shows  a  considerable  number 
of  results  by  different  experimenters  consolidated  into  a 
curve  giving  the  relation  between  current  and  efficiency, 
as  based  on  mean  spherical  candle-power. 


i6o 


THE   ART   OF   ILLUMINATION. 


The  data  for  forming  a  similar  curve  for  alternating 
arcs  are  not  available,  but  there  is  in  this  case  the  same 
sort  of  relation  between  current  and  efficiency  as  that  just 
shown.  There  is  generally  accounted  to  be  from  15  to 


Current  in  amperes. 

Fig.  53-— Relation'between  Current  and  Efficiency. 

25  per  cent,  difference  in  absolute  efficiency  in  favor  of  the 
continuous-current  arc. 

Obviously  the  open  arc  has  a  much  greater  efficiency 
than  the  enclosed  form,  but  the  very  great  intrinsic  bril- 
liancy of  the  former  is  from  the  standpoint  of  practical 
illumination  a  most  serious  drawback.  For  all  indoor 
use  and  for  much  outdoor  use  the  open  arc  must  be 
shielded  by  a  diffusing  globe,  which  to  be  effective  should 
cut  off  about  25  per  cent,  of  the  light.  Taking  this  into 
account,  the  working  efficiency  of  the  open  arc  in  the  sizes 
generally  used  is  likely  to  range  between  1.25  and  1.50 
watts  per  candle-power.  It  cannot,  however,  be  made  a 


THE   ELECTRIC   ARC   LAMP.  161 

satisfactory  illuminant  upon  any  terms  until  better  car- 
bons are  used  than  are  now  available  at  a  moderate  price 
in  this  country. 

Recently  some  very  interesting  experiments  have  been 
tried  abroad  with  carbons  impregnated  with  certain 
metallic  salts.  These  composite  carbons  have  given  ex- 
traordinary efficiency,  down  to  less  than  0.5  watt  per 
spherical  candle-power,  and  give  a  light  softer  and  less 
brilliantly  white  than  usual.  The  product  has  not  yet, 
however,  been  brought  into  commercial  form,  so  that  it  is 
too  early  to  speak  with  certainty  regarding  its  merits. 
Arcs  formed  between  two  slender  pencils  of  such  material 
as  is  used  in  the  Nernst  glower  have  also  been  tried,  and 
have  given  an  enormous  efficiency,  even  greater  than  that 
just  mentioned.  But  the  process  is  yet  far  from  being  in 
commercial  shape,  so  that  nothing  definite  can  be  judged 
as  to  its  practical  value.  The  immense  intrinsic  brilliancy 
of  such  an  arc  would  be  a  serious  difficulty  with  its  use  as 
an  illuminant. 

We  may  now  form  some  idea  of  the  relative  efficiency 
of  different  classes  of  lights.  The  annexed  table  puts  the 
facts  in  convenient  form  for  reference  : 


KIND  OF  ARC.                                 PERSPH  CP.                            REMARKS. 

Direct  current,  open  .............  .      i.o  Medium  power  arc. 

Direct  current,  shaded.  .  .  .    .......      1.3  Medium  power  arc. 

Alternating  current,  open  .........      1.7  Approximate., 

Alternating  current,  shaded  ......      2.2  Approximate. 

Direct   current,  enclosed  .........     2.4  No  outer  globe. 

Direct  current,  enclosed     ........      2.9  Clear  outer  globe. 

Direct  current,  enclosed  .........     3.3  Opal  outer  globe. 

Alternating  current,  enclosed  ......     3.0  Clear  outer  globe. 

Alternating  current,  enclosed  ......     3.6  Opal  outer  globe. 

Alternating  current,  enclosed  ......      2.5  No  outer  globe.      j 

Direct  current,  enclosed  ..........      1.9  Series    lamp,    approximate. 

In  watts  per  horizontal  candle-power  the  enclosed  arcs, 
particularly  the  alternating  ones,  do  relatively  much  bet- 


I 


m 

UNIVERSITY  ) 


162  THE   ART    OF   ILLUMINATION. 

ter  than  is  indicated  in  the  table.  And  in  comparing  arcs 
with  incandescents  it  must  be  remembered  that  the  latter, 
when  rated  like  the  above  arcs,  on  mean  spherical  candle- 
power  and  average  efficiency  while  in  use,  will  not  do 
much  better  than  4  to  4.5  watts  per  candle.  But  it  is 
adaptation  to  the  work  in  hand  that  must  determine  the 
use  of  one  or  another  illuminant. 


CHAPTER  VIII. 

SHADES    AND    REFLECTORS. 

As  has  already  been  pointed  out,  the  illuminants  in 
common  use  leave  much  to  be  desired  in  the  distribution 
of  light,  and  have,  for  the  most  part,  too  great  intrinsic 
brilliancy.  The  eye  may  suffer  from  their  use,  and  even 
if  this  does  not  occur,  the  illumination  derived  from  them 
is  less  useful  than  if  the  intrinsic  brilliancy  were  reduced.. 

Hence  the  frequent  use  of  shades  and  reflectors  in 
manifold  forms.  Properly  speaking,  shades  are  intended 
to  modify  the  light  by  being  placed  between  it  and  the 
eye,  while  reflectors  are  primarily  designed  to  modify  the 
distribution  of  the  light  rather  than  its  intensity.  Prac- 
tically the  two  classes  often  merge  into  each  other  or  are 
combined  in  various  ways. 

There  is,  besides,  a  considerable  class  of  shades  of 
alleged  decorative  qualities,  which  neither  redistribute  the 
light  in  any  useful  manner  nor  shield  the  eye  to  any 
material  degree.  Most  of  them  are  hopelessly  Philistine, 
and  have  no  aesthetic  relation  to  any  known  scheme  of 
interior  decoration.  Figs.  54  and  55,  a  stalactite  and 
globe  respectively,  of  elaborately  cut  glass,  are  excellent 
examples  of  things  to  be  shunned.  Cut  glass  is  not  at  its 
best  when  viewed  by  transmitted  light,  and  neither  dif- 
fuses nor  distributes  the  light  to  any  advantage.  Such 
fixtures  logically  belong  over  an  onyx  bar  inlaid  with 
silver  dollars,  and  to  that  class  of  decoration  in  general. 
Almost  equally  bad  are  shades  that  produce  a  strongly 


i64 


THE   ART    OF   ILLUMINATION. 


streaked  or  mottled  appearance,  like  Figs.  56  and  57. 
These  neither  stop  the  glare  from  a  too  intense  radiant 
nor  render  the  illumination  more  practically  useful  by 
improving  its  distribution.  These  shades  happen  to  be 


Figs.  54  and  55. — Cut  Glass  Stalactite  and  Globe. 

all  of  them  for  incandescent  lamps,  but  they  are  evil  in 
both  principle  and  application,  and  would  be  equally  bad 
in  connection  with  any  other  kind  of  illuminant. 

With  open  gas  flames  a  shade  may  be  of  some  use  as  a 
protection  from  draughts,  but  generally  its  purpose  is  to 


Figs.  56  and  57. — Shades  to  Avoid. 

improve  the  illumination,  and  if  it  fails  of  this  it  has  no 
excuse  for  being.  For  artistic  reasons  it  is  sometimes  de- 
sirable even  to  reduce  the  illumination  to  a  deep  mellow 
glow  quite  irrespective  of  economy,  and  in  such  case 
shades  may  be  made  ornamental  to  any  degree  and  of  any 
density  required,  or  lights  may  be  distribute^  for  purely 


SHADES   AND    REFLECTORS.  165 

decorative  purposes,  but  gaudy  spotted  and  striped  affairs, 
like  those  just  shown,  are  useless  even  for  these  ends. 

The  first  requirement  of  a  shade  is  that  it  shall  actually 
soften  and  diffuse  the  light  it  shelters.  If  it  does  not  do 
this,  no  amount  of  ornamentation  can  make  it  tolerable 
from  an  aesthetic  standpoint.  Almost  any  kind  of  orna- 
mentation is  permissible  that  does  not  defeat  this  well- 
defined  object.  Translucent  porcelain,  ground  and 
etched  glass  are  all  available  in  graceful  forms.  If  per- 
fectly plain  shades,  like  Fig.  58,  seem  too  severe,  the» 


Figs.  58,  59,  and  60. — Shades. 

those  finely  etched  in  inconspicuous  figures,  like  Figs.  59 
and  60,  may  answer  the  purpose.  The  main  thing  is  to 
conceal  the  glaring  incandescent  filament  or  mantle  so 
that  it  will  not  show  offensively  bright  spots.  Hence  the 
general  objection  to  cut  glass,  which,  if  used  at  all,  should 
for  the  display  of  its  intrinsic  beauty  be  so  arranged  that 
it  can  be  seen  by  strong  reflected  light  rather  than  by  that 
which  comes  from  its  interior. 

Thin  paper  and  fabrics  may  be  most  effectively  em- 
ployed for  shades  and  can  readily  be  made  to  harmonize 
with  any  style  of  ornamentation  or  color  scheme  that  may 
be  in  hand.  In  this  respect  such  materials  are  far  pre- 
ferable to  glass  or  porcelain,  although  more  perishable  and 
less  convenient  for  permanent  use  on  a  large  scale.  They 
also  entail  much  loss  of  light,  and  are  far  better  suited  to 
domestic  illumination  than  to  larger  installations. 


i66  THE   ART    OF   ILLUMINATION. 

The  real  proportion  of  light  cut  off  by  such  shades  has 
not,  to  the  author's  knowledge,  ever  been  accurately 
measured,  and,  indeed,  by  reason  of  the  immense  variety 
in  them,  it  would  be  almost  impossible  to  average.  It  is 
safe  to  say,  however,  that  it  is  generally  over  50  per  cent, 
although  the  light  is  so  much  softened  that  the  loss  is  not 
seriously  felt  in  reading  or  in  other  occupations  not  tax- 
ing the  eyes  severely. 

With  respect  to  porcelain  and  glass  shades  the  propor- 
tion of  light  absorbed  has  been  measured  many  times,  and 
on  many  different  kinds  of  shades,  so  that  actual,  even  if 
diverse,  figures  are  available.  The  following  table  gives 
the  general  results  obtained  by  several  experimenters  on 
the  absorption  of  various  kinds  of  globes,  especially  with 
reference  to  arc  lights: 

PER  CENT. 

Clear  glass 10 

Alabaster  glass 15 

Opaline  glass 20-40 

Ground  glass 25-30 

Opal  glass 25-60 

Milky  glass 30-60 

The  great  variations  to  which  these  absorptions  are 
subject  are  evident  enough  from  these  figures.  They 
mean,  in  the  rough,  that  clean,  clear  glass  globes  absorb 
about  TO  per  cent,  of  the  light,  and  that  opalescent  and 
other  translucent  glasses  absorb  from  15  to  60  per  cent., 
according  to  their  density.  Too  much  importance  should 
not  be  attached  to  this  large  absorption,  since  it  has  al- 
ready been  shown  that  in  most  cases,  so  far  as  useful  effect 
is  concerned,  diffusion  and  the  resulting  lessening  of  the 
intrinsic  brilliancy  is  cheaply  bought  even  at  the  cost  of 
pretty  heavy  loss  in  total  luminous  radiation. 

The  classes  of  shades  commonly  used  for  incandescent 


SHADES   AND    REFLECTORS.  167 

lamps  and  gas  lights  have  been  recently  investigated  with 
considerable  care  by  Mr.  W.  L.  Smith,  to  whom  the  au- 
thor is  indebted  for  some  very  interesting  data  on  this 
subject. 

The  experiments  covered  more  than  twenty  varieties  of 
shades  and  reflectors,  and  both  the  absorption  and  the 
redistribution  of  light  were  investigated.  One  group  of 
results  obtained  from  6-inch  spherical  globes,  intended  to 
diffuse  the  light  somewhat  without  changing  its  distribu- 
tion, was  as  follows,  giving  figures  comparable  with 
those  just  quoted : 

PER  CENT. 

Ground  glass 24.4 

Prismatic  glass 20. 7 

Opal  glass 32.2 

Opaline  glass 23.0 

The  prismatic  globe  in  question  was  of  clear  glass,  but 
with  prismatic  longitudinal  grooves,  while  the  opal  and 
opaline  globes  were  of  medium  density  only. 

Etched  glass,  like  Figs.  59  and  60,  has  considerably 
more  absorption  than  any  of  the  above,  the  etching  being 
optically  equivalent  to  coarse  and  dense  grinding.  Their 
diffusion  is  less  homogeneous  than  that  given  by  ordi- 
nary grinding,  so  that  they  may  fairly  be  said  to  be  un- 
desirable where  efficiency  has  to  be  seriously  considered. 

A  plain,  slender  canary  stalactite  behaved  like  the  globes 
as  respects  distribution,  and  showed  just  the  same  absorp- 
tion as  the  ground  glass  globe,  i.  e.,  24.4  per  cent.,  but 
permitted  an  offensively  brilliant  view  of  the  filament 
within. 

Another  group  of  tests  had  to  do  with  reflecting  shades 
designed  to  throw  light  downward,  in  some  cases  giving  a 
certain  amount  of  transmitted  light,  in  others  being  really 
opaque.  The  characteristics  of  some  common  forms  of 


i68  THE   ART   OF   ILLUMINATION. 

such  shades  are  plainly  shown  by  the  curves  of  light  dis- 
tribution made  with  the  shades  in  place.  Figs.  61  and 
62  show  two  thoroughly  typical  examples  of  these  shades. 
Fig.  6 1  is  the  ordinary  enameled  tin  8-inch  shade,  green 


Fig.  61. — Conical  Shade.  Fig.  62. — Fluted  Cone. 

on  the  outside  and  brilliant  white  within,  a  form  too  often 
used  over  desks.  Fig.  62  is  almost  as  common,  being  a 
fluted  porcelain  6-inch  shade,  used  in  about  the  same  way 
as  Fig.  61.  Figs.  63  and  64  give  the  respective  vertical 
distributions  produced  by  these  two  shades,  the  outer 


Fig.  63. — Distribution  from  Fig.  64. — Distribution  from 

Conical  Shade.  Fluted  Cone. 

circles  showing  for  reference  the  nominal  i6-cp  rating. 
The  porcelain  not  only  gives  a  more  uniform  reflection 
downwards,  but  transmits  some  useful  light  outwards. 
The  case  as  between  it  and  the  tin  shade  of  Figs.  61  and 
63,  which  gives  a  strong  but  narrow  cone  of  light  down- 
ward, may  be  tabulated  as  follows : 


SHADES   AND   REFLECTORS.  169 

8-INCH  TIN     6-INCH  FLUTED 
ENAMELED.          PORCELAIN. 

Mean  spherical  candle-power 8.12  9.89 

Maximum  candle-power 29.49  18.15 

Horizontal  candle-power o.oo  5.26 

Absorption,  per  cent 28.1  12.4 

The  absorption  is,  of  course,  based,  as  elsewhere,  on  the 
mean  spherical  candle-power.  Of  these  two  shades  the 
porcelain  one  is  considerably  the  better  for  practical  pur- 
poses. Although  it  gives  a  somewhat  smaller  maximum 
candle-power  directly  below  the  lamp,  it  gives  a  much 


Fig.  65. — Shallow  Cone.  Fig.  66. — McCreary  Shade. 

larger  well-lighted  area,  and  is  for  every  reason  to  be  pre- 
ferred. The  unaltered  vertical  distribution  of  an  incan- 
descent lamp  is  given  in  the  curve  shown  in  Chapter  VI., 
and  that  curve  was  from  the  same  lamp  used  in  testing 
these  shades. 

It  should  be  noted  that  the  relations  of  these  two  forms 
would  not  be  materially  altered  if  they  were  of  appro- 
priate size  and  were  applied  to  Welsbach  burners,  the  dis- 
tribution of  light  from  which  bears  a  rather  striking  re- 
semblance to  that  from  an  incandescent  lamp.  The  tin 
shade  gives  too  much  the  effect  of  a  bright  spot  to  be 
really  useful  for  most  purposes.  If  such  a  concentrated 
beam  is  desired  it  is  far  better  obtained  by  other  and  more 
perfect  methods. 


170 


THE   ART    OF   ILLUMINATION. 


Figs.  65  and  66  show  two  other  forms  of  reflecting 
shade  in  somewhat  common  use,  the  former  designed  to 
give  the  light  a  general  downward  direction,  the  latter 
to  produce  a  strong  and  uniform  downward  beam.  Fig. 
65  is  a  6-inch  fluted  porcelain  shallow  cone,  while  Fig.  66 
is  the  well-known  and  excellent  McCreary  shade,  7-inch. 
They  are  intended  for  widely  different  purposes,  which 
come  out  clearly  in  the  curves  of  distribution,  Figs.  67 
and  68. 

The  flat  porcelain  cone,  Fig.  67,  merely  gathers  a  con- 
siderable amount  of  light  that  would  ordinarily  be  thrown 


VERTICAL  OK    Of HORIZOJJIAL 


Figs.  67  and  68. — Curves  of  Distribution. 

upward,  and  scatters  it  outwards  and  downwards.  It 
has  a  generally  good  effect  in  conserving  the  (light,  and 
whether  applied  to  an  incandescent  lamp  or  a  Welsbach 
deflects  downward  a  good  amount  of  useful  illumination. 
The  McCreary  shade,  on  the  other  hand,  is  deliberately 
intended  to  give  a  rather,  concentrated  beam,  softened, 
however,  by  the  ground  glass  bottom  of  the  shade.  As 
Fig.  68  shows,  it  accomplishes  this  result  quite  effectively, 
giving  a  powerful  and  uniform  downward  beam.  The 


SHADES   AND   REFLECTORS.  171 

annexed  table  shows  in  a  striking  manner  the  difference  in 
the  two  cases : 

FLAT  PORCE- 
LAIN CONE.       McCREARY. 

Mean  spherical  candle-power 9.84  7.50 

Maximum  candle-power 15.72  42.72 

Horizontal  candle-power 13-94  2.29 

Absorption,  per  cent 12.8  33.5 

The  small  absorption  in  the  first  instance  is  merely  due 
to  the  fact  that  the  shade  is  not  reached  by  any  con- 
siderable portion  of  the  light,  while  the  large  absorption 
in  the  later  case  only  indicates  that  nearly  the  whole  body 
of  light  is  gathered  by  reflection,  and  sent  out  through  a 
diffusing  screen. 

The  porcelain  cone  is  irremediably  ugly,  but  a  less 
offensive  shade  having  the  same  general  properties  may 
often  be  put  to  a  useful  purpose.  The  McCreary  shade 
is  purely  utilitarian,  but  neat,  and  does  its  work  well  in 
producing  a  strong,  directed  illumination — a  bit  too  con- 
centrated, perhaps,  for  ordinary  desk  work,  but  most  use- 
ful for  work  requiring  unusually  bright  light.  Of  fancy 
shades  modified  in  various  ways  there  are  a  myriad, 
usually  less  good  than  the  examples  here  shown. 

In  cases  where  concentration  of  light  downwards  along 
the  axis  of  the  lamp  is  desirable,  rather  efficient  results  are 
attained  by  combining  lamp  and  reflector,  that  is,  by  shap- 
ing the  bulb  of  the  lamp  itself  so  that  when  the  part  of  it 
nearest  the  socket  is  silvered  on  the  outside  it  shall  form 
an  effective  reflector  of  proper  shape.  Obviously  when 
the  lamp  burns  out  or  dim  the  whole  combination  becomes 
useless,  in  which  respect  the  device  is  less  economical  than 
an  ordinary  lamp  in  a  carefully  designed  reflecting  shade 
like  the  McCreary.  On  the  other  hand,  the  reflector 
lamps  are,  on  the  whole,  somewhat  more  efficient  during 


172  THE   ART    OF   ILLUMINATION. 

their  useful  life,  and  for  general  purposes  of  illumination 
are  much  less  obtrusive. 

In  such  lamps  the  bulb,  instead  of  being  pear-shaped, 
is  spherical  or  spheroidal,  with  the  upper  hemisphere  sil- 
vered, the  silvering  being  protected  by  a  coat  of  lacquer. 
The  filament  usually  has  several  convolutions  of  rather 
small  radius,  so  as  to  bring  as  large  a  proportion  of  the 
incandescent  filament  as  possible  near  to  the  center  of  the 
bulb.  A  filament  so  disposed  throws  an  unusual  propor- 
tion of  the  light  upwards  and  downwards  when  the  lamp 
is  mounted  with  its  axis  vertical,  but,  of  course,  at  the 
expense  of  the  horizontal  illumination. 

It  has  sometimes  been  proposed  to  use  a  filament  so 
shaped  in  an  ordinary  bulb  for  the  purpose  of  throwing  a 
strong  light  downwards.  Evidently,  however,  such  a 
lamp  throws  just  as  much  light  upwards  toward  the 
opaque  socket  and  the  ceiling  as  downwards  toward  the 
tip.  Hence  there  is  certainly  no  gain  in  general  illumi- 
nation, and  no  important  gain  in  the  downward  illumina- 
tion unless  the  lamp  is  mounted  with  a  reflector.  Fig.  69 
shows  in  outline  a  nominal  5O-cp.  spherical  bulb  lamp  with 
a  silvered  upper  hemisphere,  and  Fig.  70  the  curve  of  light 
distribution  in  a  vertical  plane.  The  maximum  down- 
wards is  about  75  candle-power,  while  a  fair  amount  of 
light  is  thrown  out  laterally  up  to  30  degrees  from  the 
horizontal.  The  dotted  curve  shows  the  distribution  with 
the  silvered  bulb,  the  solid  curve  the  distribution  after  the 
silvering  had  been  removed. 

Fig.  71  shows  a  common  type  of  reflecting  bulb  lamp. 
As  usually  employed,  the  lower  part  of  the  bulb  is  ground 
so  as  to  soften  and  diffuse  the  light.  The  general  charac- 
teristics of  reflectors  and  their  field  of  usefulness  have 
already  been  discussed,  so  that  it  is  here  hardly  necessary 


SHADES   AND    REFLECTORS. 


'73 


to  say  more  than  to  reiterate  that  whenever  a  downward 
light  is  desired  reflector  lamps  may  furnish  a  most  useful 
means  of  getting  it.  Their  weak  point,  as  generally 
made,  is  a  tendency  to  produce  a  spotted  effect  through 
too  great  concentration  of  the  light  along  the  central  axis. 
If  a  projector  effect  is  the  one  thing  desired  silvered  bulb 
lamps  are  not  the  best  means  of  getting  it,  and  for  their 


Figs.  69  and  70. — Reflector  Lamp  and  Its  Distribution. 

greater  usefulness  as  illuminants  they  should  be  so  con- 
structed as  to  give  a  nearly  uniform  hemispherical  dis- 
tribution. Most  of  the  silvered  bulb  lamps  on  the  market 
fail  to  do  this.  And  it  should  be  noted  that  since  the 
whole  lamp  is  lost  when  the  filament  breaks,  there  is  a 
strong  temptation  to  fit  such  lamps  with  a  low  efficiency 
filament  in  order  to  give  a  longer  life.  A  properly  de- 
signed lamp  of  this  class  planned  for  hemispherical  dis- 
tribution could  not  fail  to  be  of  much  use  in  general 
illumination,  and  could  be  produced  at  a  very  small  extra 
cost  above  standard  lamps. 


174,  THE   ART    OF   ILLUMINATION. 

For  various  illuminants  shades  require  to  be  somewhat 
modified  in  form,  and  an  enormous  variety  of  shades  and 
reflectors  are  on  the  market,  of  which  those  here  described 
may  merely  serve  as  samples.  Shading  the  radiant, 
whatever  it  may  be,  is  a  simple  matter,  and  so  is  the  use 


Fig.  71. — Reflector  Lamp. 

of  a  pure  reflector  (best  made  of  silvered  glass)  to  direct 
the  light  in  any  particular  direction.  But  the  commonest 
fault  of  powjerful  radiants,  as  we  have  already  seen,  is  too 
great  intrinsic  brilliancy,  which  calls  for  diffusion,  and 
good  diffusion  without  great  loss  of  light  is  difficult  of 
attainment,  particularly  if  at  the  same  time  there  is  need 
of  redistributing  the  light  so  as  to  strengthen  the  illumina- 
tion in  any  particular  direction. 

By  far  the  most  successful  solution  of  this  troublesome 


SHADES   AND    REFLECTORS.  175 

problem  is  found  in  the  so-called  holophane  globes,  de- 
vised a  few  years  ago  by  MM.  Blondel  and  Psaroudaki, 
and  now  in  somewhat  extensive  use  both  here  and  abroad. 
The  general  principle  employed  by  these  physicists  was 
to  construct  a  shade  of  glass  so  grooved  horizontally  as 
to  form  the  whole  shade  of  annular  prisms.  These  are 
not  formed  as  in  a  lighthouse  lens,  to  act  entirely  by  re- 
fraction, because  in  the  attempt  to  bend  the  rays  through 
a  large  angle  by  refraction  alone  there  is  a  large  loss  by 
total  reflection. 

The  prisms  of  the  holophane  globe  are  relieved,  as  it 
were,  at  certain  points,  so  that  rays  which  need  to  be  but 
little  deflected  are  merely  refracted  into  the  proper  direc- 
tion, while  those  that  must  be  greatly  bent  to  insure  the 
proper  direction  are  affected  by  total  reflection.  This 
combination  of  refracting  and  reflecting  prisms  in  the 
same  structure  accomplished  the  efficient  redistribution 
of  the  light  in  a  very  perfect  manner.  The  diffusion  re- 
mained to  be  effected,  and  the  means  adopted  was  to  form 
the  interior  of  the  globe  into  a  series  of  rather  fine,  deep, 
rounded,  longitudinal  grooves. 

The  total  result  is  a  great  reduction  of  the  intrinsic  bril- 
liancy, coupled  with  almost  any  sort  of  distribution  re- 
quired, the  total  loss  of  light  meanwhile  being  less  than 
in  any  other  known  form  of  diffusing  shade  or  reflector. 
Fig.  72  shows  in  detail,  considerably  magnified,  the 
structure  of  the  holophane  prisms  and  the  combination  of 
refraction  and  reflection  that  is  their  characteristic  fea- 
ture. Here  the  ray  A  is  merely  refracted  in  the  ordinary 
way,  emerging  with  a  strong  downward  deflection  from 
the  prism  face  in  the  direction  A1.  Ray  B  Bl  is  totally 
reflected  at  the  face  &1,  and  then  refracted  outwards  at  b. 
C  is  strongly  refracted  and  emerges  from  the  surface  c, 


I76 


THE   ART   OF   ILLUMINATION. 


while  D  D1  is  refracted  at  entrance,  totally  reflected  at  d1, 
and  again  refracted  at  emergence  from  d. 

The  net  result  is  to  keep  in  this  particular  form  of  prism 
surface  nearly  all  the  rays  turned  downward  below  the 
horizontal.  Obviously  other  prismatic  forms  might  be 


Fig.  72. — Section  of  Holophane  Globe. 

employed,  which  would  give  a  very  different  final  distri- 
bution, but  the  principles  involved  are  the  same. 

Fig.  73  shows,  likewise  on  a  greatly  enlarged  scale,  the 
interior  fluting  which  accomplishes  the  necessary  diffu- 
sion of  light.  The  ray  .a  is  here  split  up  into*  a  reflected 
component,  afterwards  refracted — b,  e,  i,  g  and  a 
purely  refracted  component,  b,  c,  d.  The  shape  of  the 
flutings  is  such  as  by  this  means  to  secure  very  excellent 
diffusion  at  a  very  small  total  loss  of  light.  The  inner 
and  outer  groovings  being  at  right  angles  produce  a 
somewhat  tessellated  appearance,  but  aside  from  this  the 
surface  is  quite  uniformly  illuminated. 

These   holophane  globes   are   made   for  all   kinds   of 


SHADES   AND    REFLECTORS.  177 

radiants,  but  are  most  commonly  applied  to  Welsbach 
gas  burners  and  to  incandescent  electric  lights.  Evi- 
dently the  shape  of  both  grooves  and  globe  must  vary 
with  the  purpose  for  which  the  shade  is  desired,  which 
results  in  a  very  large  number  of  forms  from  which  a 
selection  may  be  made  for  almost  any  variety  of  illumina- 
tion. 

It  should  be  noted  that  these  holophane  shades  both 
diffuse  and  redistribute  the  light  in  a  very  thorough  man- 
ner. Speaking  generally,  they  are  of  three  distinct 
classes.  The  first  is  laid  out  according  to  the  general 
principles  of  Fig.  72,  and  is  intended  to  direct  most  of  the 


Fig-  73. — Diffusing  Curves  of  Holophane. 

light  downwards,  serving  the  same  end  as  a  reflector,  but 
giving  at  the  same  time  the  needful  diffusion  without  the 
use  of  a  ground  or  frosted  globe.  The  general  results  are 
strikingly  shown  in  Fig.  74,  which  gives  a  very  graphic 
idea  of  what  such  a  globe  actually  does. 

The  second  class  of  globes  has  for  its  purpose  a  fairly 
uniform  distribution  of  the  light,  mainly  below  the  hori- 
zontal, and  it  is  intended  for  ordinary  indoor  lighting, 
where  a  particularly  strong  light  in  any  one  direction  is 
quite  needless.  Its  effect  is  shown  in  Fig.  75.  The  third 
general  form  of  holophane  globe  is  designed  for  the 


178  THE   ART    OF   ILLUMINATION. 

especial  purpose  of  throwing  a  strong  light  out  in  a  nearly 
horizontal  direction,  and  is  shaped  so  as  thus  to  redis- 
tribute the  light,  putting  it  where  it  is  most  useful  for 
such  work  as  street  lighting,  large  interiors,  and  the  like. 
The  effect  produced  is  admirably  shown  by  Fig.  76.  The 


Fig.  74. — Holophane,  Downward  Distribution. 

shapes  of  globes  shown  in  these  last  three  figures  are  those 
intended  for  mantle  burners. 

In  general,  the  device  enables  a  good  degree  of  diffu- 
sion to  be  secured  together  with  almost  any  peculiarity  of 
distribution  that  could  be  wanted,  and  with  a  degree  of 
efficiency  that  is  unequaled  by  any  known  system  of 
shades  or  reflectors,  unless  it  be  the  Fresnel  lighthouse 
lenses. 

One  does  not  generally  get  such  a  combination  of  good 
qualities  without  certain  disadvantages  that  must  be  taken 
in  partial  compensation.  In  the  holophane  system  the 


SHADES   AND    REFLECTORS.  179 

weak  point  is  dirt.  The  doubly  grooved  surface  makes 
an  excellent  dust  catcher,  and  a  layer  of  dust  can  easily 
be  accumulated  quite  sufficient  to  cut  down  the  efficiency 
very  seriously.  And,  moreover,  a  hasty  dab  writh  a  rag 
does  not  clean  a  holophane  globe;  it  must  be  gone  over 
carefully  and  thoroughly.  When  kept  clean,  the  globes 
actually  will  do  just  what  is  claimed  for  them,  and  are  not 


Fig.  75. — Holophane,  General  Distribution. 

at  all  a  merely  theoretical  development  excellent  only  on 
paper;  but  they  must  be  kept  clean,  and  should  not  be  used 
where  they  cannot  or  will  not  receive  proper  attention. 

This  is  probably  the  chief  reason,  aside  from  the  extra 
cost,  why  such  globes  have  not  been  more  extensively 
used  for  street  lighting,  to  which  their  power  of  redis- 
tributing the  light  in  the  most  useful  direction  admirably 
fits  them.  The  results  obtained  in  tests  of  these  globes 
are  so  striking  as  to  merit  examination  in  some  detail. 

In  spite  of  the  trouble  from  dust,  the  holophane  globes 
have  come  into  considerable  use  for  street  lighting  in  some 
European  cities,  notably  Munich,  where  several  thousand 
have  been  used  on  Welsbach  street  lamps  for  several  years 


i8o  THE    ART    OF    ILLUMINATION. 

past.  The  net  results  is  reported  to  be  exceedingly  good, 
although  the  amount  of  labor  involved  must  be,  from  an 
American  standpoint,  very  considerable.  Breakage  in 
this  case  is  reported  at  about  10  per  cent,  per  annum. 

If  this  device  could  be  successfully  applied  to  arc  lamps 
for  street  lighting,  a  very  valuable  redistribution  of  the 


Fig.  76. — Holophane, Outward  Distribution. 

light  might  be  effected,  but  certain  obstacles  seem  to  be 
interposed  on  account  of  the  shifting  of  the  arc  as  the 
carbons  are  consumed.  With  a  focusing  form  of  lamp 
this  trouble  would  be  averted,  but  such  lamps  have  never 
yet  come  into  considerable  use  in  this  country.  With  en- 
closed arcs,  however,  it  should  be  possible  to  use  these 
globes  with  a  very  fair  degree  of  success. 

Tests  of  holophane  globes  on  incandescent  lamps  show 
in  a  thoroughly  typical  manner  the  effect  of  their  peculiar 
structure  in  diffusing  and  redistributing  the  light.  For 
example,  Fig.  77  exhibits  the  distribution  produced  by  a 
holophane  of  the  "  stalactite  "  shape  designed  to  throw 
the  light  downward.  It  does  this  very  effectively,  giving 
less  of  a  spotted  effect  than  any  of  the  reflecting  devices, 
and  at  the  same  time  diffusing  the  light  from  the  bare  fila- 
ment very  thoroughly. 


SHADES   AND    REFLECTORS.  181 

The  light  absorbed  by  this  stalactite  was  only  14.6  per 
cent.,  which  is  considerably  less  than  would  be  lost  by  any 
equally  effective  device  for  diffusion  alone,  to  say  noth- 
ing of  the  matter  of  redistribution.  And  this  figure  for 
absorption  is  probably  increased  by  the  deflection  of  so 
much  light  downwards,  since  similar  shades  designed  to 


Figs.  77  and  78. — Distribution  Curves. 

throw  more  light  laterally  give  materially  smaller  absorp- 
tion. 

From  a  considerable  number  of  tests  the  holophane 
shades  appear  to  average  about  12  per  cent,  absorption 
when  clean.  That  is,  they  actually  transmit  very  nearly 
as  much  light  as  clear  glass  globes  that  have  no  value  in 
redistributing  or  diffusing  the  light.  As  to  their  use- 
fulness in  indoor  work  there  is  little  doubt. 

For  street  lighting  they  are  capable  of  immensely  im- 
proving the  distribution  of  the  light,  but  the  matter  of 
keeping  them  clean  is  serious.  Their  greatest  chance  for 
practical  usefulness  would  be  in  connection  with  arc  lamps 


i8a  THE   ART    OF   ILLUMINATION. 

if  the  details  of  such  an  application  should  be  thoroughly 
worked  out. 

Fig.  78  shows  the  distribution  curve  from  a  holophane 
stalactite  designed  to  produce  a  more  general  distribution 
than  Fig.  77,  the  absorption  in  this  instance  being  only 
10.8  per  cent.  The  principle  employed  in  these  shades 
would  readily  lend  itself  to  the  distribution  of  light  in 
special  directions  for  special  purposes,  but  the  variety  of 
work  in  which  light  has  to  be  directed  is  so  great  that 
special  problems  are  best  treated  by  the  use  of  reflectors 
which  are  comparatively  cheap  and  can  readily  be  adapted 
to  any  required  form. 


CHAPTER   IX. 

DOMESTIC     ILLUMINATION. 

THE  lighting  of  houses  is  a  most  interesting  and  gen- 
erally neglected  branch  of  illumination.  Artificial  light 
has  been  distinctly  a  luxury  until  within  comparatively 
recent  times,  and  in  domestic  lighting  there  has  not 
been  the  same  pressure  of  commercial  necessity  which 
has  resulted  in  the  general  efforts  to  illuminate  other 
buildings.  Indeed,  until  within  half  a  century  there  was 
very  little  effort  at  really  good  illumination  in  the  home, 
everyone  depending  on  portable  lights  which  could  be 
brought  directly  to  bear  upon  the  work  in  hand,  gas, 
which  provides  fixed  radiant  points,  being  confined  to 
large  cities,  and  in  these  to  houses  of  the  better  class. 
Even  at  the  present  time  very  little  pains  is  taken  to  ar- 
range the  lighting  in  a  systematic  and  efficient  manner. 

The  comparatively  small  areas  to  be  lighted  in  dwell- 
ings, the  small  need  for  extremely  intense  light,  and  the 
very  discontinuous  character  of  the  need  for  any  light  at 
all,  render  domestic  lighting  rather  a  problem  by  itself. 
Of  ordinary  illuminants  all  may  be  freely  used  for  such 
work,  save  arc  lamps  and  very  powerful  gas  lamps,  such 
as  the  large  regenerative  burners  and  the  most  powerful 
incandescent  mantles. 

Arcs  are  of  very  unnecessary  power,  hence  most  un- 
economical, and  are  generally  so  unsteady  as  to  be  most 
trying  to  the  eyes.  In  the  home,  as  a  general  thing,  one 
does  not  keep  the  eyes  fixed  in  any  definite  direction,  as 

183 


i84  THE   ART   OF   ILLUMINATION. 

one  would  if  working  steadily  by  artificial  light,  so  that 
far  more  than  usual  care  must  be  taken  to  avoid  intense 
and  glaring  lights.  Therefore,  arcs  are  most  objection- 
able, and  the  gas  lights  of  high  candle-power  equally  so, 
particularly  as  the  latter  throw  out  a  prodigious  amount 
of  heat  and  burn  out  the  oxygen  of  the  air  very  rapidly. 

As  to  other  illuminants,  the  main  point  is  to  choose 
those  of  low  intrinsic  brilliancy,  or  to  keep  down  the  in- 
trinsic brilliancy  by  adroit  and  thorough  shading.  Any- 
thing over  two  or  three  candle-power  per  square  inch  it 
is  well  to  avoid  as  needlessly  trying  to  the  eyes  without 
any  compensating  advantage  save  economy,  which  can 
better  be  secured  in  other  ways. 

Aside  from  the  physiological  side  of  the  matter,  very 
bright  lights  seldom  give  good  artistic  results  or  show 
an  interior  at  anything  like  its  true  value.  Of  the  com- 
mon illuminants,  gas  and  incandescent  lamps  are  those 
generally  most  useful,  while  petroleum  lamps  and 
candles  are  even  now  auxiliaries  by  no  means  to  be  de- 
spised. Professor  Elihu  Thomson  once  very  shrewdly 
remarked  to  the  writer  that  if  electric  lights  had  been  in 
use  for  centuries  and  the  candle  had  been  just  invented, 
it  would  be  hailed  as  one  of  the  greatest  blessings  of  the 
century,  on  the  ground  that  it  is  absolutely  self-con- 
tained, always  ready  for  use,  and  perfectly  mobile. 

Now,  gas  and  incandescents,  while  possessing  many 
virtues,  lack  that  of  mobility.  They  are  practically  fixed 
where  the  builder  or  contractor  found  it  most  con- 
venient to  install  them,  for  while  tubes  or  wires  can  be 
led  from  the  fixtures  to  any  points  desired,  these  strag- 
gling adjuncts  are  sometimes  out  of  order,  often  in  the 
way,  and  always  unsightly.  Besides,  the  outlets  are 
often  for  structural  reasons  in  inconvenient  locations, 


DOMESTIC  ILLUMINATION.  185 

and  their  positions  need  to  be  chosen  very  carefully  if 
artistic  effects  are  at  all  to  be  considered;  so  that  while 
these  lights  are  the  ordinary  basis  of  illumination  wher- 
ever they  are  available,  lamps  and  candles,  which  can  be 
put  where  they  are  wanted  and  not  necessarily  where 
some  irresponsible  workman  chose  to  locate  them,  are 
often  most  useful  additions  to  our  resources. 

In  domestic,  as  in  other  varieties  of  interior  illumina- 
tion, two  courses  are  open  to  the  designer  of  the  illumi- 
nation. In  the  first  place,  he  can  plan  to  have  the  whole 
space  to  be  lighted  brought  uniformly  or  with  some  ap- 
proximation to  uniformity,  above  a  certain  brilliancy 
more  or  less  approximating  the  effect  of  a  room  receiv- 
ing daylight  through  its  windows.  Or,  throwing  aside 
any  purpose  to  simulate  daylight  in  intensity  or  dis- 
tribution, he  can  put  artificial  light  simply  where  it  is 
needed,  merely  furnishing  such  a  groundwork  of  general 
illumination  as  will  serve  the  ends  of  art  and  con- 
venience. 

While  the  first  method  is  for  purely  utilitarian  pur- 
poses sometimes  necessary,  it  is  always  uneconomical 
and  generally  most  inartistic  in  its  results.  Its  sin 
against  economy  is  in  furnishing  a  great  deal  of  light 
which  is  not  really  needed,  while  in  so  doing  it  usually 
sends  light  in  directions  where  it  deadens  shadows,  blurs 
contrasts,  and  illuminates  objects  on  all  sides  but  the 
right  one.  The  second  method  is  the  one  uniformly  to 
be  chosen  for  domestic  lighting,  from  every  point  of 
view. 

In  electric  lighting  the  most  strenuous  efforts  are  con- 
stantly being  made  to  improve  the  efficiency  of  the  in- 
candescent lamps  by  a  few  per  cent.,  and  an  assured  gain 
of  even  10  per  cent,  would  be  heralded  by  such  a  fanfare 


186  THE   ART    OF   ILLUMINATION. 

of  advertising  as  has  not  been  heard  since  the  early  days 
of  the  art.  Yet  in  lighting  generally,  and  domestic 
lighting  in  particular,  a  little  skill  and  tact  in  using  the 
lights  we  now  have  can  effect  an  economy  far  greater 
than  all  the  material  improvements  of  the  last  twenty 
years.  The  fundamental  rule  of  putting  light  where  it 
is  most  useful,  and  concentrating  it  only  where  it  is 
needed,  is  one  too  often  forgotten  or  unknown.  If 
borne  in  mind,  it  not  only  reduces  the  cost  of  illumina- 
tion, but  improves  its  effect. 

In  applying  this  rule  in  practice,  one  of  the  first  things 
which  forces  itself  upon  the  attention  is  the  fact  that  the 
conditions  can  seldom  be  met  by  the  consistent  use  of 
lights  of  one  uniform  intensity,  or  one  uniform  char- 
acteristic as  regards  the  distribution  of  the  light  around 
the  radiant.  Even  one  kind  of  illuminant  is  sometimes 
an  embarrassing  condition.  Both  the  kind  and  quantity 
of  the  illumination  must  be  adjusted  to  the  actual  re- 
quirements, if  real  efficiency  is  to  be  secured. 

As  has  already  been  shown,  the  effective  illumination 
depends  upon  two  factors — the  actual  power  of  the 
radiant  in  candles  or  other  units,  and  the  nature  of  the 
surroundings,  which  determine  the  character  and 
amount  of  the  diffuse  reflection  which  re-enforces  the 
direct  light.  If  the  radiant  in  a  closed  space  furnishes 
a  certain  quantity  of  light,  L,  then  the  strength  of  the 
illumination  produced  at  any  point  within  the  space  will 
depend,  if  the  walls  are  non-reflecting,  simply  on  the 
amount  of  light  received  from  the  radiant,  in  accordance 
with  the  law  of  inverse  squares.  If  the  walls  reflect,  then 
the  total  illumination  at  any  point  will  be  that  received 
directly,  L.  and  in  addition  a  certain  amount  k  L  (where 
k  is  the  coefficient  of  reflection),  once  reflected,  a  further 


DOMESTIC  ILLUMINATION.  187 

amount  k2  L  twice  reflected,  and  so  forth.  The  total 
illuminative  effect  will  then  be: 

Z(i  +£  +  ^  +  £3+.  .  .  &) 

As  k  is  obviously  always  less  than  unity,  this  series  is 
convergent  upon  the  limiting  value  -M  t  _  /,  li  which 

expresses  the  relative  effect  of  the  walls  in  re-enforcing 
the  light  directly  received  from  the  radiant. 

It  is  clear  from  the  values  of  k  already  given  for  vari- 
ous surfaces,  that  such  assistance  may  be  of  very  great 
practical  importance.  A  simple  experiment  showing  the 
value  of  the  light  diffusely  reflected  is  to  read  at  some 
little  distance  from  the  radiant  in  a  room  having  light 
walls,  and  then  to  cut  off  the  direct  rays  by  a  screen 
close  to  the  radiant  and  just  large  enough  to  shade  the 
book.  If  the  conditions  are  favorable,  the  amount  of 
diffused  illumination  will  be  somewhat  startling.  A 
repetition  of  the  experiment  in  a  room  with  dark  walls 
will  exhibit  the  reverse  condition  in  a  most  striking 
manner. 

In  practice  the  interior  finish  of  dwelling  houses  is 
highly  heterogeneous,  the  walls  being  tinted  and  broken 
with  doors  and  hangings,  the  ceiling  being  often  of  an- 
other color,  and  the  floors  covered  with  colored  rugs  or 
carpets,  and  generally  provided  with  furniture  at  least 
as  dark  as  the  walls.  The  floor  is  in  point  of  fact  the 
least  important  surface  from  the  standpoint  of  illumina- 
tion, for  it  not  only  carries  the  furniture,  but  from  its 
position  cannot  diffuse  light  in  any  useful  direction.  So 
far  as  it  is  concerned,  only  the  small  terms  in  k2  and 
higher  powers  enter  the  general  equation,  since  the  illumi- 
nation diffused  from  below  is  not  of  any  practical  account 
whatever. 


i88  THE   ART    OF   ILLUMINATION. 

A  good  idea  of  the  practical  amount  of  help  received 
from  diffusion  may  be  gained  by  computing  the  effect 
for  various  values  of  k.  The  following  table  shows  the 
results  for  values  of  k  between  .05  and  .95. 


t  «  \ 

(*-*) 


-95  ............................................    20. 

.90  ..........................   .................  10. 

.85  ............................................  6.66 

-.80  .............................................  5.00 

•  75  .............................................  4-00 

.70  ...  .........................................  3-33 

.65  ...........................................  2.85 

.60  ...................................  ,  ........  2.50 

.55  .............     ...................     ..........       2.22 

.50  ............................................       2.00 

.81 

.66 

•  53 
.42 

•33 

•  25 


.11 

•  05 


These  values  show  the  great  difference  between  good 
and  poor  diffusing  surfaces  in  their  practical  effect. 
Reference  to  the  table  already  given  shows  that  ordi- 
nary wall  surfaces  give  values  of  k  ranging  from  about 
.60  down  to  .10  or  less.  These  are  likely  to  be  reduced 
by  the  gradual  absorption  of  dust  at  the  surface,  but  it 
is  quite  within  bounds  to  say  that  the  effective  illumina- 
tion in  a  room  may  be  nearly  or  quite  doubled  by  the 
light  diffused  from  the  walls. 

On  the  contrary,  the  ceiling  is  a  very  important  con- 
sideration, for  the  light  diffused  downward  is  highly 
valuable.  Paneled  or  vaulted  ceilings  are  notorious  in 
their  bad  effect  upon  the  illumination.  If  used  at  all, 


DOMESTIC  ILLUMINATION.  189 

they  should  be  employed  with  full  knowledge  of  the 
fact  that  they  quite  effectively  nullify  all  attempts  at 
brilliant  general  illumination,  and  when  considerations 
of  harmony  permit,  ceilings  ought  to  be  very  lightly 
tinted. 

As  to  the  walls  themselves,  wainscoting  and  dark  soft- 
finished  papers  absorb  light  very  strongly,  and  render 
lighting  difficult,  while  the  white-painted  wood  and 
light  papers  freely  used  in  Colonial  houses  produce 
exactly  the  reverse  effect.  The  character  of  interior 
finish,  being  determined  by  the  contemporaneous 
fashion,  can  of  course  seldom  be  really  subordinated  to 
the  matter  of  illumination,  which  affects  only  personal 
comfort;  but  in  planning  a  scheme  of  decoration  it  is 
necessary  to  bear  in  mind  that  the  darker  the  general 
effect  the  more  light  should  be  provided. 

The  outlets  for  gas  and  electricity  provided  for  and 
quite  adequate  to  light  a  brightly-finished  house,  will 
prove  entirely  insufficient  if  a  scheme  of  decoration  in 
dark  colors  be  afterward  carried  out,  so  that  it  is  the  part 
of  wisdom  to  arrange  the  original  outlets  to  meet  the 
worst  probable  conditions  for  lighting.  This  will  gen- 
erally mean  arranging  for  about  double  the  minimum 
amount  of  illumination  wanted  on  the  hypothesis  of 
strong  diffusion  from  the  walls. 

If  conditions  demand  or  fashion  dictates  any  attempt 
at  very  bright  illumination,  a  sort  of  simulated  daylight, 
all  matters  relating  to  diffusion  are  of  very  serious  im- 
port. Fortunately,  such  is  not  the  usual  case.  Where 
the  main  purpose  is  that  already  strongly  urged,  of 
merely  furnishing  such  illumination  as  is  necessary  for 
practical  or  artistic  purposes,  there  need  be  no  effort  at 
uniform  intensity  of  light  or  at  making  dark  corners 


1 90  THE   ART   OF   ILLUMINATION. 

brilliant;  and,  while  the  aid  of  favorable  diffusion  is  still 
important  in  reducing  the  total  amount  of  artificial  light 
furnished,  it  no  longer  so  completely  controls  the  situa- 
tion. 

With  the  data  now  at  hand  we  can  form  a  fairly  defi- 
nite idea  of  the  quantity  of  light  which  must  generally 
be  provided.  One  can  get  at  the  approximate  facts  by 
considering  the  amount  of  light  that  must  be  furnished 
in  a  room  of  given  size  to  bring  the  general  illumina- 
tion up  to  a  certain  value.  The  particular  value  as- 
sumed must  depend  upon  the  purpose  for  which  the 
room  is  to  be  lighted.  For  instance,  since  i  candle-foot 
is  an  amount  which  enables  one  to  read  rather  easily,  let 
us  assume  that  we  are  to  furnish  in  a  room  20  ft.  square 
and,  say,  10  ft.  high,  a  minimum  of  i  candle- foot. 

To  start  with,  we  must  make  some  assumption  as  to 
the  amount  gained  by  diffusion  from  ceiling  and  walls. 
This,  in  a  concrete  case,  we  can  make  an  educated  guess 
at  from  the  data  already  given.  In  general,  Wybauw 
found  that  in  moderate-sized  rooms  the  diffusion  in- 
creased the  effective  value  of  the  radiant  50  per  cent., 
which,  as  it  agrees  pretty  closely  with  our  own  values, 
taking  into  account  a  light  ceiling,  we  will  use  for  the 
present  purpose. 

Let  the  assumed  radiant  be  at  r,  Fig.  79,  and  at  a 
height  of  6'  6"  above  the  floor.  Now  draw  an  imagi- 
nary plane  a  b  at  a  height  of  2!  6"  above  the  floor,  and 
take  this  as  the  surface  to  be  illuminated.  If  r  is  in  the 
center  of  the  room,  the  greatest  distance  from  r  to  a  cor- 
ner of  the  plane  a  b  will  be  \/ '216  ft.  =  14.7  ft.  Each 
candle-power  at  r  must  be  reduced  proportionately,  so 
that  i  candle  at  r  would  give  1-216  candle-foot  at  the 
point  in  question.  According  to  our  hypothesis,  diffu- 


DOMESTIC  ILLUMINATION.  191 

sion  aids  by  50  per  cent.,  so  that  instead  of  requiring  216 
candle-power  to  give  i  candle-foot  in  the  remotest  cor- 
ner, the  real  amount  would  be  144  candle-power,  which 
would  be  handily  furnished  by  a  cluster  of  nine  i6-cp 
incandescent  lamps.  The  result  would  be  a  room  quite 
brilliantly  lighted,  for,  except  very  near  the  walls,  the 
illumination  would  be  much  in  excess  of  i  candle-foot, 
rising  to  4  or  5  candle-feet  upon  the  plane  of  lighting 
under  and  near  the  lights. 

Such  an  arrangement  of  the  lights  is,  however,  un- 
economical in  the  extreme,  since  the  distant  corners  are 


4 


Fig.  79- — Vertical  Section  of  Room. 

illuminated  at  a  very  great  disadvantage.  Fig.  80  shows 
the  advantage  gained  by  a  rearrangement.  Here  the 
room  is  divided  by  imaginary  lines  into  four  ro-ft. 
squares,  and  in  the  center  of  each  of  these  is  a  light  6'  6" 
above  the  floor,  as  before.  Now,  if  a  corner  of  the  plane 
of  lighting,  as  E,  receives  i  candle-foot,  the  requirements 
are  fulfilled.  But  E  is  distant  from  D  just  about  8  ft., 
from  C  and  B  almost  exactly  16  ft.  and  from  A  less  than 
22  ft.  It,  therefore,  receives,  neglecting  A,  for  each 
candle-power  at  D  1-64  candle-foot,  ajid  for  each  at  C 


I92 


THE   ART   OF   ILLUMINATION. 


and  B  a  total  of  1-128  candle-foot,  or,  allowing  for  diffu- 
sion, 1-43  and  1-87,  respectively  (nearly),  so  that  it  at 
once  becomes  evident  that  four  32-cp  lamps  are  more 
than  sufficient  to  do  the  work. 

Taking  A  into  account,  four  25~cp  lamps  would  al- 
most suffice,  but  obviously  the  maximum  illumination 
is  perceptibly  lowered.  It  would  be  a  maximum  at  the 
center,  and  for  32-cp  lamps  would  there  amount  to  2 
candle-feet.  A  still  further  subdivision  would  lead  to 


1 


-20- 


At, 


D 


X\ 


Fig.  80.— Floor  Plan.  Fig.  8 1.— Floor  Plan. 

still  better  distribution  from  .the  point  of  view  of 
economy,  and,  indeed,  something  can  still  be  gained  by 
a  further  redistribution  of  the  light.  For,  with  lights 
arranged  as  in  Fig.  81,  at  the  center  and  on  the  circle 
inscribed  in  the  room  in  question,  five  2O-cp  lamps 
would  very  closely  fulfill  the  conditions,  reducing  the 
total  amount  of  light  required  to  meet  the  assumed  con- 
dition from  144  to  100  candle-power  in  all. 

Obviously,  with  a  fixed  minimum  illumination  and  no 
other  requirement,  the  conditions  of  economy  will  be 
most  nearly  met  by  a  nearly  uniform  distribution  of  the 
minimum  intensity  required.  There  is,  however,  a 
limit  to  practical  subdivision  in  limited  areas,  such  as 


DOMESTIC  ILLUMINATION.  193 

rooms.  In  the  case  of  large  buildings,  as  we  shall  pres- 
ently see,  one  can  easily  figure  out  the  illumination  on 
the  basis  just  taken,  but  in  domestic  lighting  we  have 
to  deal  with  a  very  limited  number  of  radiants,  at  least 
in  considering  gas  and  electricity. 

By  far  the  best  results  are  attained  by  providing  a 
very  moderate  general  illumination  and  then  superpos- 
ing upon  it  strong  local  illumination  for  special  pur- 
poses. For  example,  in  most  rooms  better  practical  re- 
sults than  those  of  Fig.  81  would  be  reached  by  follow- 
ing the  same  arrangement,  but  using  four  i6-cp  or  even 
four  8-cp  lamps  and  one  32-cp  lamp,  the  latter  being 
placed  near  the  point  where  the  strongest  illumination 
is  required.  The  result  would  be  to  give  the  extreme 
corners  all  the  light  they  really  need,  and  to  provide 
plenty  of  light  where  it  is  of  most  practical  value. 

The  same  rules  apply  to  the  use  of  gas  or  other 
illuminants,  always  bearing  in  mind  that  the  total 
amount  of  light  required  is  strongly  affected  by  the  hue 
of  the  walls,  and  that  the  principal  radiant  should  be 
placed  where  it  will  do  the  most  good.  Illumination 
thus  regulated  is  both  safer  physiologically  and  far  more 
efficient  in  use  of  the  material  than  any  attempt  at  uni- 
form distribution  over  the  entire  area. 

One's  choice  of  illuminants  must  obviously  be  gov- 
erned by  the  question  of  availability.  Incandescent 
electric  lamps  easily  hold  the  first  place  when  economy 
is  not  the  first  consideration,  by  reason  of  their  being 
quite  steady,  giving  out  little  heat,  and  in  no  way  vitiat- 
ing the  atmosphere.  They  should  always,  however,  be 
furnished  with  ground  bulbs,  or  so  shaded  as  to  reduce 
their  otherwise  very  high  intrinsic  brilliancy. 

Next  in  order  of  desirability  unquestionably  comes 


i94  THE   ART   OF   ILLUMINATION. 

gas.  Used  with  the  incandescent  mantle  burner,  it  is 
the  cheapest  known  illuminant  for  domestic  purposes, 
but  in  this  form  is  too  bright  for  anything  except  the 
principal  radiant.  Mantle  burners  should  always  be 
shaded,  both  to  reduce  the  intrinsic  brilliancy  and  to 
modify  the  greenish  cast  of  the  light  which  otherwise  is 
highly  objectionable.  Ordinary  gas  jets,  if  the  pressure 
be  fairly  steady,  give  a  good  subordinate  illumination. 

Lamps  and  candles  have  strong  merits  for  particular 
pr.i-poses,  but  are  inferior  for  general  work.  The  former 
are  often  used  with  good  effect  to  furnish  the  principal 
radiant,  which  may  be  re-enforced  by  small  gas  jets. 
Candles,  on  the  other  hand,  are  extremely  useful  for 
partial  and  subsidiary  illumination,  since  they  are  the 
only  available  source  of  small  intensity  unless  one  goes 
to  considerable  trouble  in  wiring  for  tiny  electric  bulbs, 
which  are  better  adapted  to  purely  decorative  purposes 
than  to  the  regular  work  of  illumination. 

From  this  general  basis  of  facts  we  can  now  take  up 
the  practical  and  concrete  side  of  domestic  lighting. 

As  to  the  distribution  of  the  lights  required  for  in- 
terior illumination,  one  must  be  guided  by  the  intensity 
which  is  necessary.  The  examples  already  given  show 
the  general  character  of  the  problem.  The  laws  upon 
which  the  solution  depend  may  be  formulated  as  fol- 
lows :  If  we  write  L  for  the  required  or  existing  inten- 
sity of  illumination  in  candle-feet  at  any  point,  C  for  the 
candle-power  of  the  radiant,  and  d  for  the  distance  in  feet 
from  that  radiant,  then: 


If  the  point  in  question  receives  light  from  more  than 


DOMESTIC  ILLUMINATION.  195 

one  radiant,  the  illuminative  effects  must  be  summed, 
and,  if  the  radiants  are  equal, 


C  = 


L  is  of  course  in  candle-feet  and  C  in  candle-power. 
In  these  expressions  no  account  is  taken  of  the  varying 
angles  of  incidence  of  the  light  received  from  the  sev- 
eral radiants.  In  principle,  L  =  — — — ,  where  i  is  the 

angle  of  incidence;  in  other  words,  the  illumination  de- 
creases as  it  becomes  oblique. 

In  certain  cases  account  must  be  taken  of  this  fact, 
but  since,  as  a  rule,  objects  to  be  lighted  are  oblique  to 
the  plane  of  illumination,  and  cos  i  is  small  only  in  case 
of  rather  distant  lights,  of  which  the  entire  effect  is 
small,  and  since  the  diffused  light  cannot  be  reckoned 
with,  having  no  determinate  direction,  the  question  of 
obliquity,  particularly  when  the  radiants  are  numerous 
and  well  distributed,  has  seldom  to  be  dealt  with.  It  is 
rendered  the  more  uncertain  by  the  notorious  inequality 
of  the  distribution  of  the  light  from  ordinary  illuminants, 
and  it  must  be  remembered  that  the  whole  aspect  of  the 
matter  is  changed  by  the  use  of  reflectors. 

In  ordinary  interior  illumination  one  constantly  meets 
limitations  imposed  by  structural  or  artistic  considera- 
tions. For  example,  we  have  already  seen  that  the 
arrangement  shown  in  Fig.  81  was  highly  desirable  far 
economic  reasons.  The  five  lamps  dangling  by  cords 


196  THE   ART   OF   ILLUMINATION. 

or  rods,  however  ornamental,  from  the  ceiling  of  a  room 
20  ft.  square,  might  be  tolerated  in  an  office,  but  would 
be  quite  inadmissable  in  a  drawing  room.  For  domestic 
lighting  one  is  practically  confined  to  chandeliers,  side 
lights,  or  ceiling  lights.  The  latter  have  been  consider- 
ably used  of  late,  sometimes  with  beautiful  effects;  some- 
times unwrisely. 

To  examine  the  effect  of  ceiling  lights  on  the  situation, 
refer  to  Fig.  82,  which  shows  the  same  room  as  Fig.  79. 


IT 

i 

'T 

v|  1 

if 

i  i 

111 

i 
i 

i 

Fig.  82.— Location  of  Ceiling  Lights. 

Assuming  the  same  general  conditions,  let  us  find 
the  illumination  at  a  point  p  in  the  plane  of  illumina- 
tion when  given  by  a  light  r  in  the  old  position,  and  a 
ceiling  light  /,  6  ins.  below  the  ceiling.  The  light 

being  of  16  candle-power,  the  light  at  p  is  L  —  --  =  .39 

41 

candle-foot,  when  the  lamp  is  at  r,  or    Z  =  —  =  .21 

74 

when  the  lamp  is  at  r',  close  to  the  ceiling,  neglecting 
diffused  light. 

In  a  room  very  bright  with  white  paint  or  paper,  hav- 
ing, for  example,  k  =  .60  and  (  -    [—\  =  2.50,  the  total 


DOMESTIC  ILLUMINATION.  197 

illumination  would  be  .39  +  .97  =  1.36,  and  since  the 
diffusion  does  not  materially  change  with  the  position 
of  the  light,  the  illumination  in  the  second  case  would 
be,  roughly,  .21  +  .97  =  1.18;  in  other  words,  the 
change  in  position  of  the  light  would  make  but  a  small 
change  in  the  intensity  of  the  illumination. 

There  is  evidently  some  error  made  in  assuming  that 
diffusion  increases  the  illumination  by  a  certain  ratio, 
and  Wybauw's  hypothesis  of  replacing  the  diffused  light 
by  an  imaginary  radiant  directly  above  the  real  radiant 
involves  the  same  error.  It  is  probably  nearer  the 
truth  to  assume,  in  case  of  an  apartment  having  several 
radiants,  that  the  total  illumination  at  any  point  is  that 
due  to  the  lights  severally,  plus  a  uniform  illumination, 
due  to  diffusion  and  proportional  to  k  and  C. 

The  practical  upshot  of  the  matter,  however  one  may 
theorize  on  the  rather  hazy  data,  is  that  shifting  the 
lights  in  a  room  from  their  usual  height  to  the  ceiling 
does  not  affect  the  illumination  seriously  if  the  walls  and 
ceiling  diffuse  strongly,  while  if  they  are  dark  the 
change  is  decidedly  unfavorable.  This  does  not,  how- 
ever, imply  that  ceiling  lights  should  not  be  used  in 
dark-finished  rooms,  although  it  is  very  plain  that  if  they 
are  so  used  the  lamps  should  be  provided  with  reflectors, 
or  themselves  form  reflectors,  as  in  some  lamps  recently 
introduced. 

If  the  walls  have  a  very  low  coefficient  of  diffusion  it 
is  obvious  that  all  light  falling  upon  them  is  nearly 
wasted,  at  least  from  the  standpoint  of  illumination,  and 
therefore  the  economic  procedure  is  to  deflect  this  light 
so  that  instead  of  falling  upon  the  walls  it  shall  be  di- 
rected upon  the  plane  of  illumination,  which  is  chosen 
to  represent  the  average  height  from  the  floor  at  which 


198  THE   ART    OF   ILLUMINATION. 

are  the  things  to  be  illuminated.  If  reflectors  or  their 
equivalents  are  skillfully  applied,  the  radiants,  for  the 
purpose  in  hand,  are  nearly  or  quite  doubled  in  intensity, 
so  that  there  is  a  good  opportunity  for  efficient  lighting. 
But  these  reflecting  media  must  be  used  with  caution 
to  avoid  the  appearance  of  beams  giving  definite  bright 
areas,  and  by  far  the  best  results  may  be  obtained  by 
using  ground  or  frosted  bulbs  in  such  cases.  So  far  as 
economy  of  light  is  concerned,  reflectors  can  be  advan- 
tageously used  wherever  the  effective  reflection  exceeds 
the  total  diffusion  coefficient  of  the  walls.  For  example, 
with  a  hemispherical  reflector  having  a  coefficient  of 
reflection  of  .70,  the  hemispherical  intensity  of  the  radiant 
is  1.70  C,  assuming  a  spherical  distribution  of  the  light. 
This  value  corresponds,  so  far  as  the  plane  of  illumination 
is  concerned,  with  a  diffusion  of  k  =  .40,  which  signifies 
that,  except  in  very  light-finished  rooms,  the  radiant  is 
used  more  efficiently  by  employing  a  reflector  than  by 
trusting  to  the  really  very  serviceable  diffusion  from  the 
walls. 

But  if  the  reflector  aperture  be  as  great  as  a  hemi- 
sphere, there  is  still  some  material  aid  to  be  gained  by 
diffusion.  In  the  case  already  discussed  in  Fig.  81,  if 
reflector  lamps  were  used,  five  i6-cp  lamps  would  meet 
the  requirements,  and  would  fall  but  a  trifle  below  the 
requirements  even  if  used  as  ceiling  lamps. 

It  is  safe  to  say  that  by  the  use  of  reflector  lamps  the 
work  of  effective  lighting  from  the  ceiling  is  made  fairly 
easy,  if  the  ceilings  are  of  ordinary  height.  Without 
reflectors  it  is  a  method  greatly  lacking  in  economy. 

The  use  of  side  lights  close  to  the  wall,  or  on  short 
brackets,  is  preferable  to  lighting  from  the  ceiling  when 
the  latter  is  high,  or  when,  as  often  happens,  strong 


DOMESTIC  ILLUMINATION. 


199 


local  illumination  is  needed.  Reflector  lamps  may  here 
again  be  used  with  very  great  effect  if  the  walls  are  at 
all  dark  in  tone.  Fig.  83  gives  in  diagram  the  simplest 
arrangement  of  such  lamps.  We  may  assume  their 
height  as  a  trifle  less  than  in  the  case  of  the  suspended 
lights,  say,  3  ft.  above  the  plane  of  illumination,  and 
that  they  are  equipped  with  reflectors  giving  a  hemi- 
spherical distribution  of  light.  In  Fig.  83  the  positions 


-20— 


IQ: 


Fig.  83.— Side  Lights. 

of  the  lamps  are  indicated  by  black  dots,  as  before.  It 
is  evident  that  the  corners  will  be  the  points  of  mini- 
mum illumination,  and  that  in  the  central  part  of  the 
room  the  lighting  will  be  rather  weak,  although,  on  the 
whole,  the  distribution  of  light  will  be  good.  With 
help  from  diffusion  to  the  extent  assumed  in  the  last 
example,  four  2O-cp  reflector  lamps  would  do  the  work, 
while  with  dark  walls  the  case  would  call  for  four  32-cp 
lamps. 

Now,  summarizing  our  tentative  arrangements  of 
light,  it  appears  that  to  illuminate  a  room  20  ft.  square 
and  10  ft.  high  on  the  basis  of  a  minimum  of  i  candle- 
foot,  will  require  from  80  to  144  effective  candle-power, 


200  THE   ART   OF   ILLUMINATION. 

according  to  the  arrangement  of  the  lights,  if  the  finish 
is  light,  and  half  as  much  again,  at  least,  if  the  finish  is 
dark.  The  floor  space  being  400  sq.  ft.,  it  appears  that 
the  illumination  is  on  the  basis  of  about  3  to  5  sq.  ft.  per 
effective  candle-power.  The  former  figure  will  give 
good  illumination  under  all  ordinary  conditions;  the  lat- 
ter demands  a  combination  of  light  finish  and  very  skill- 
fully arranged  lights. 

For  very  brilliant  effects,  no  more  than  2  sq.  ft.  per 
candle  should  be  allowed,  while  if  economy  is  an  object, 
i-cp  to  4  sq.  ft.  will  furnish  a  very  good  groundwork  of 
illumination,  to  be  strengthened  locally  by  a  drop-light 
or  reading  lamp.  The  intensity  thus  deduced  we  may 
compare  to  advantage  with  the  results  obtained  by  vari- 
ous investigators,  reducing  them  all  to  such  terms  as  will 
apply  to  the  assumed  room  which  we  have  had  under 
discussion. 

Just  deduced i-cp  per  3     sq.  ft. 

Uppenborn i-cp  per  3.6  sq.  ft. 

Piazzoli i-cp  per  3. 5  sq.  ft. 

Fontaine i-cp  per  7.0  sq.  ft.   (approximation). 

In  very  high  rooms  the  illumination  just  indicated 
must  be  materially  increased,  owing  to  the  usual  neces- 
sity for  placing  the  lamps  rather  higher  than  in  the  case 
just  given,  and  on  account  of  the  lessened  aid  received 
from  diffuse  reflection.  The  amount  of  this  increase  is 
rather  uncertain,  but  in  very  high  rooms  it  would  be 
wise  to  allow  certainly  i-cp  for  every  2  sq.  ft.,  and  some- 
times, as  in  ballrooms  and  other  special  cases  requiring 
the  most  brilliant  lighting,  as  much  as  i-cp  per  square 
foot. 

On  the  other  hand,  in  most  domestic  lighting,  the 
amount  of  lighting  needed  may  be  reduced  by  a  little 


DOMESTIC  ILLUMINATION.  201 

tact.  Ordinary  living  rooms,  such  as  parlors,  libraries, 
and  the  like,  do  not  require  to  be  uniformly  and  brightly 
lighted  in  most  cases.  It  is  sufficient  if  there  is  ample 
light  throughout  the  main  portion  of  the  room. 

A  groundwork  illumination  of  0.5  candle-foot  over  the 
whole  room,  plus  a  working  illumination  of  i  to  1.5 
candle-foot  in  addition  over  a  part  of  the  room,  gives  an 
excellent  result.  This  is  something  the  result  that 
would  be  reached  in  Fig.  Si  by  using  a  32-cp  central 
lamp  and  four  lo-cp  lamps  for  the  rest  of  the  room. 
Dining  rooms  need  ample  light  upon  the  table,  but  do 
not  in  the  least  require  illumination  of  equal  power  in 
the  remote  corners.  Sleeping  and  dressing  rooms  do 
not  require  strong  light  so  much  as  well-placed  light. 
A  bedroom  of  the  dimensions  we  have  been  discussing 
could  be  very  effectively  lighted  with  three  or  four  i6-cp 
lamps,  provided  they  were  placed  where  they  would  do 
the  most  good. 

To  go  into  detail  a  little,  perhaps  the  most  important 
rule  for  domestic  lighting  is  never  to  use,  indoors, 
an  incandescent  or  other  brilliant  light,  unshaded. 
Ground  or  frosted  bulbs  are  particularly  good  when  in- 
candescents  are  used,  and  opal  shades,  or  holophane 
globes,  which  also  reduce  the  intrinsic  brilliancy,  are 
available  with  almost  any  kind  of  radiant.  Ornamental 
shades  of  tinted  glass  or  of  fabrics  are  exceedingly  use- 
ful now  and  then,  when  arranged  to  harmonize  with 
their  surroundings. 

In  incandescent  lighting  the  lamps  may  be  placed  in 
any  position.  With  gas  or  other  flame  radiants  ceiling 
lights  are  not  practicable.  As  to  the  intensity  of  the 
individual  radiants,  considerable  latitude  may  be  given. 
In  many  instances,  incandescents  or  gas  or  other  lights 


202  THE   ART    OF   ILLUMINATION. 

of  as  low  as  8  to  lo-cp  are  convenient,  while  for  stronger 
illumination  radiants  of  15  to  2O-cp  reduce  the  cost  of 
installation,  and  for  special  purposes  lights  of  30  to 
5o-cp,  incandescents  or  incandescent  gas  lamps,  are 
most  useful.  To  get  a  clear  view  of  the  application  of 
the  principles  here  laid  down  it  will  be  well  to  take  up 
in  some  detail  the  illumination  of  a  typical  modern  house 
in  its  various  particulars. 

Beginning  at  the  porch,  the  light  here  is  of  purely 
utilitarian  value.  One  32-cp  incandescent  or  its  equiva- 
lent in  gas  would  generally  be  sufficient,  enclosed  in  an 
inoffensive  antique  iron  lantern.  Fig.  84  shows  a  fine 
specimen  of  eighteenth  century  ironwork. 

Hall. — Assumed  dimensions,  15  ft.  x  20  ft.,  finished  in 
some  combination  like  ebony  and  old  yellow.  Gen- 
erally the  staircase  forbids  the  effective  use  of  a  chande- 
lier, and  lights  can  best  be  put  upon  wrought-iron  side- 
brackets.  The  lighting  required  for  the  300  sq.  ft.  is 
not  strong,  and  four  i6-cp  or  eight  8-cp  units,  arranged 
on  two  or  four  brackets,  would  give  all  the  illumination 
ever  required.  Fig.  85,  an  antique  two-light  iron  bracket, 
will  give  a  useful  hint.  Lanterns  are  often  used  here, 
but  they  generally  are  in  the  way. 

Library. — Assumed,  20  ft.  x  20  ft.,  in  mahogany  and 
dull  green.  The  form  of  the  room  and  the  presence  of 
bookcases  complicate  the  illumination.  The  bookcases, 
unless  so  much  space  is  absolutely  necessary,  should  not 
be  carried  to  the  ceiling.  The  conditions  are  severe. 
With  incandescents  very  good  results  could  be  reached 
by,  say,  twelve  8-cp  ground-bulb,  reflector  lamps, 
worked  into  the  frieze,  and  a  reading  lamp  of  not  less 
than  32-cp,  as  a  drop-light,  preferably  with  a  tinted  holo- 
phane  or  other  globe.  With  gas,  or  with  high  bookcases, 


DOMESTIC  ILLUMINATION. 


203 


old  brass  side-brackets  on  each  side  of  the  fireplace  or  else- 
where opposite  the  cases,  carrying  in  all  the  equivalent 


Fig.  84. — Iron  Porch  Lantern. 

of  eight  i6-cp  lamps,  and  a  mantle  burner,  well  shaded, 
as  a  reading  lamp,  would  answer.  In  fact,  very  good 
work  could  be  gotten  from  two  shaded  mantle  burners 
as  side  lights. 

Reception  Room. — Assumed,  15  ft.  x  15  ft.,  cream  and 
rose,  or  similar  light  finish.  Strong  light  is  not  needed 
here,  and  an  ornate  gilt  brass  chandelier,  carrying  four 


204 


THE   ART   OF   ILLUMINATION. 


8-cp  or  lo-cp  lamps  or  their  equivalent  should  prove 
ample. 

Music  Room. — Assumed,  20  ft.  x  25  ft.,  in  white  and 
gold  or  the  like.     For  musical  purposes  two  32-cp  lights 


-Tl 


Fig.  85.— Iron  Wall  Bracket. 

in  holophane  globes,  carried  as  piano  lamps,  shaded,  and 
for  general  illumination  about  twelve  8-cp  lights,  carried 
in  groups  on  elaborate  gilt  bronze  brackets  or  sconces. 
The  arrangement,  of  course,  hinges  on  the  position  of 
windows,  etc.,  and  since  such  a  room  is  often  used  as  a 


DOMESTIC  ILLUMINATION. 


205 


Fig.  86.— Gilt  Bronze  Bracket. 

ballroom,  in  case  of  electric  lighting   provision  should 
be  made  for  replacing  the  8-cp  by  32-cp  lamps.     With 


2o6  THE   ART   OF   ILLUMINATION. 


Fig.  87.— Gilt  Bronze  Bracket. 

gas,  the  fixtures  should  be  planned  so  as  to  provide  ad- 
ditional lamps.     Figs.  86  and  87  show  two  examples  of 


DOMESTIC  ILLUMINATION. 


207 


fine  eighteenth  century  bras  de  cheminee  well  adapted  to 
cases  like  the  present. 

Dining  Room. — Assumed,  15  ft.  x  20  ft.,  in  dark  an- 
tique oak  or  mahogany  and  tapestries  or  other  dark  wall 


Fig.  88. — Wrought-Iron  Bracket. 

finish.  Here  ceiling  or  frieze  lamps  are  in  place,  one  or 
the  other,  according  to  the  nature  of  the  finish.  Eight 
8-cp  reflector  lamps,  ground,  worked  into  the  decora- 
tion, would  give  a  good  groundwork,  backed  up  by,  say, 


2o8  THE   ART   OF   ILLUMINATION. 

six  more  8-cp  or  lo-cp  ground-bulb  lamps,  on  wrought- 
iron  brackets,  of  which  Fig.  88  gives  an  excellent  an- 
tique specimen,  flanking  the  mantle,  or,  for  a  yet  better 
artistic  effect,  by  shaded  candelabra  upon  the  table  itself. 
Using  gas,  one  would  almost  be  driven  to  an  elabora- 
tion of  the  side  brackets,  or  to  a  chandelier,  too  often  an 
abomination,  and  always  difficult  to  make  artistic  in 
such  a  place. 

Billiard  Room. — Assumed,  15  ft.  x  20  ft.  Dull  reds 
and  greens  in  finish.  Lighting  here  must  be  utilitarian. 
It  requires  four  32-cp  lamps  bearing  directly  upon  the 
table.  Incandescents  or  mantle  burners  in  holophane 
globes,  or  with  slightly  translucent  reflectors,  answer 
the  purpose  well. 

SLv  Bedrooms. — Assumed,  15  ft.  x  15  ft.,  finished  in 
cream  or  other  light  paint  and  with  rather  light  walls. 
In  the  smaller  rooms,  two  i6-cp  lights  bearing  upon  the 
dressing  table  are  ample,  and  in  the  larger  rooms  these 
with  an  additional  bracket,  carrying  another  i6-cp  lamp, 
are  all  that  would  be  required. 

Two  Dressing  Rooms. — Assumed,  10  ft.  x  15  ft.,  in 
light  finish,  like  the  chambers.  Two  i6-cp  lamps,  bear- 
ing on  the  dressing  table,  will  do  the  work  well. 
Brackets  here  and  in  chambers  should  generally  be  of 
gilt  brass. 

Three  Bathrooms. — Assumed,  8  ft.  x.  10  ft.,  in  white 
and  Delft  blue  or  the  like.  One  i6-cp  light,  carried  on 
bracket,  is  sufficient. 

Three  Servants'  Rooms. — Assumed,  10  ft.  x  15  ft. 
Light  finish.  One  i6-cp  lamp,  bracketed,  near  dressing 
table. 

Kitchen. — Assumed,  15  ft.  x  15  ft.  Light  wood  and 
paint.  Two  i6-cp  lamps. 


DOMESTIC  ILLUMINATION. 


209 


Pantry. — Assumed,  10  ft.  x  15  ft.     One  i6-cp. 

Back  hall,  laundry,  and  cellar  would  be  lighted  with 
8-cp  lamps,  in  all  to  the  number  of  about  ten.  Upstairs 
halls,  three  i6-cp. 

This  programme  is  merely  intended  as  a  hint  about 
the  requirements,  and  while  it  is  laid  out  for  a  fairly 
large  house,  containing  twenty  rooms  and  three  baths, 
its  details  will  furnish  suggestions  applicable  to*  many 
places.  In  closing,  it  is  worth  mentioning  that  where 
incandescents  are  available,  an  8-cp  lamp  of  the  reflector 
variety  should  be  placed  in  the  ceiling  of  every  large 
closet,  and  controlled  by  a  switch  from  the  room  or  by 
an  automatic  switch,  turning  it  on  when  the  door  is  fully 
opened. 

The  lighting  just  described  may  be  summarized  as 
follows : 


ROOM. 

8-CP. 

i6-cp. 

32-CP. 

SQ.  FT. 
PER  CP. 

REMARKS. 

Hall  

8 

4.7 

12 

I 

3.1 

8-cp  reflector  lamps 

Reception  room  

4 

7.0 

Music  room  .    .  . 

12 

2 

3.O 

Dining  room  

14 

2.7 

Eight  reflector  lamps 

4 

2.3 

32-cp  with  reflectors 

Porch  

I 

Bedrooms  (6)  
Dressing  rooms  (2)  
Servants'  rooms  (3).  .  .  . 
Bathrooms    (3) 

14 

4 
3 

•5 

7.0 
4-7 
9.4 
5O 

Kitchen  ) 

q 

Pantry     j" 
Halls       ) 

IO 

3 

Cellar      J"  

Closets  (4)   

A 

Reflector  lamps 

Total     

64 

30 

8 

210  THE   ART   OF   ILLUMINATION. 

The  noticeable  thing  about  this  table  is  the  large 
number  of  8-cp  lamps.  These  are  for  the  purpose  of 
giving  good  distribution  of  light  in  the  rooms  where  it  is 
most  necessary.  The  total  is  equivalent  to  78  i6-cp 
lamps,  by  no  means  a  large  installation  for  a  house  of 
this  size.  In  using  gas,  mantle  burners  should  be  used 
where  32-cp  lamps  are  indicated.  These  should  always 
be  given  pinkish  or  yellowish  shades,  to  kill  the  green- 
ish tinge  of  the  light.  Pink  glass  shades,  or,  better, 
holophane  globes,  are  useful,  or  very  diaphanous  orna- 
mental fabric  shades,  lightly  dyed  with  erythrosine, 
aurine,  or  saffronine.  The  former  is  rather  fugitive,  al- 
though perhaps  the  best  in  tint.  In  a  room  with  red  walls 
of  almost  any  shade,  the  diffused  light  partially  corrects 
the  greenish  tint  of  the  radiant,  but  the  light  itself  is  too 
bright  to  go  without  shading  in  any  event.  Mantle 
burners  greatly  economize  the  use  of  gas,  and  when 
properly  shaded  may  be  advantageously  used  almost 
anywhere,  since  they  use  just  about  the  same  amount  of 
gas  as  ordinary  burners  and  give  about  three  times  as 
much  light.  They  are  much  too  powerful  to  give  the 
best  artistic  results,  however,  unless  very  cautiously 
used.  In  applying  them  to  a  case  such  as  that  we  have 
just  been  considering,  they  should  be  regarded  as 
equivalent  to  two  i6-cp  incandescents,  for,  while  really 
somewhat  brighter  than  this  suggestion  would  indicate, 
a  single  radiant  is  less  effective  than  two,  each  of  half 
the  given  power. 


CHAPTER    X. 

LIGHTING    LARGE    INTERIORS. 

THIS  branch  of  illumination  differs  from  ordinary 
domestic  lighting  in  several  important  particulars.  In 
the  first  place,  the  aid  received  from  diffusion  from  the 
walls  is  much  less  than  in  the  case  of  smaller  rooms,  as 
has  already  been  indicated.  The  experiments  of  Fon- 
taine indicate  that  within  moderate  limits  the  light  re- 
quired is  determined  by  the  volume  of  the  space  to  be 
illuminated,  rather  than  by  the  floor  space.  That  is,  in 
a  given  room,  doubling  the  height  of  the  ceiling  should 
double  the  light  required  for  proper  illumination. 

Since,  however,  the  only  physical  effect  of  the  in- 
creased height  is  to  increase  the  mean  distances  of  the  dif- 
fusing surfaces  from  the  radiants  and  hence  slightly  to 

diminish .    the     significant    ratio   I -;),  the     change 

could,  in  point  of  fact,  alter  only  that  part  of  the  total 
illumination  due  to  diffused  light,  provided  that  with  in- 
creased height  of  ceiling  the  radiants  are  not  themselves 
raised.  Hence  only  in  the  case  of  walls  capable  of  very 
brilliant  diffusion  can  the  variation  due  to  increased 
dimensions  alone  approach  the  magnitude  indicated  by 
Fontaine's  empirical  rule,  which,  however,  possesses  the 
merit  of  causing  one  to  err  in  the  right  direction  and  to 
give  ample  illumination. 

In  large  and  high  rooms  there  is  a  strong  tendency 
to  increase  the  height  of  the  radiants  above  the  plane  of 


212  THE   ART   OF   ILLUMINATION. 

illumination,  especially  in  case  of  using  chandeliers, 
and  this  is  perhaps  an  important  factor  in  the  rule  afore- 
said. Obviously  in  increasing  the  distance  of  the 
radiants  one  decreases  the  direct  illumination  approxi- 
mately in  the  ratio  of  the  inverse  squares  of  the  dis- 
tances, and  does  not  materially  improve  the  diffusion. 

Therefore  the  illumination  falls  off  seriously.  In  a 
large  and  high  hall  lights  arranged  in  the  ceiling  or  as 
a  frieze,  while  often  giving  admirable  effects,  are  quite 
uneconomical,  and  should  be  used,  if  at  all,  with  a  full 
appreciation  of  this  fact.  If  for  artistic  or  other  reasons 
the  lights  must  be  placed  high,  reflector  lamps,  or  their 
equivalent,  are  strongly  to  be  recommended. 

In  large  buildings,  too,  the  quantity  of  light  required 
is  subject  to  enormous  variation,  according  to  the  pur- 
poses to  which  the  building  is  devoted,  and  whether  the 
whole  interior  must  for  artistic  reasons  be  illuminated. 
In  a  ballroom  an  effect  of  great  brilliancy  is  generally 
aimed  at,  while  a  room  of  equal  size  used  as  a  factory 
needs  strong  illumination  only  where  it  will  facilitate  the 
work. 

Again,  in  very  large  rooms  the  power  of  the  indi- 
vidual radiants  can  advantageously  be  increased,  and 
some  sources  of  light  inadmissible  in  domestic  lighting 
— such  as  arc  lamps  and  large  regenerative  gas  burners, 
or  to  be  used  only  with  caution,  like  mantle  gas  burners 
— may  be  used  very  freely. 

But  in  large  buildings,  as  elsewhere,  the  fundamental 
purpose  of  the  lighting  is  to  produce  a  certain  intensity 
at  the  plane  of  illumination,  which  in  such  work  should 
be  assumed  about  three  feet  above  the  floor.  The  abso- 
lute illumination  required  may  vary  greatly,  over  a 
range,  in  fact,  as  great  as  from  half  a  candle-foot  to  two 


LIGHTING    LARGE    INTERIORS. 


213 


candle-feet  or  even  more,  but  the  lighting  may  properly 
be  calculated  from  an  assumed  value,  just  as  in  the  case 
already  discussed. 

For  purposes  of  discussion,  we  may  first  consider  a 
hall  100  ft.  long  by  30  ft.  high  by  50  ft.  wide.  The  plane 
of  illumination  will  then  have  an  area  of  5000  sq.  ft.,  and 
the  total  volume  is  150,000  cu.  ft.  And  for  simplicity 


--  41- 


*-  -151  -! 


r —->•'• 


Fig.  89.— Plan  of  Hall. 

we  will  assume  i  candle-foot  as  the  minimum  intensity 
to  be  permitted  in  any  part  of  the  space.  Fig.  89  shows 
the  plan  of  this  assumed  space.  We  will  first  take  up 
the  case  of  suspended  radiants,  which  is  the  most  usual 
method  of  treating  such  a  problem. 

Obviously  in  a  room  of  the  shape  given  a  single 
radiant  is  out  of  the  question,  on  the  ground  of  econ- 
omy, since  in  meeting  the  requirement  of  a  given  mini- 
mum of  illumination  the  most  economical  arrangement 
is  that  which  exceeds  this  minimum  at  the  fewest  points 
possible.  Two  radiants  give  a  possible  solution,  and  are 
worth  a  trial.  Clearly  they  must  be  located  on  the 
major  axis  of  the  room  A  B;  but  since  a  corner,  as  E,  is 
the  most  unfavorable  spot  to  light,  the  radiants  must  be 
placed  well  toward  the  ends  of  the  room.  We  will  as- 


2i4  THE   ART   OF   ILLUMINATION. 

sume  their  height  as  15  ft.  above  the  floor,  and  12  ft. 
above  the  plane  of  illumination. 

Now  the  best  position  of  a  given  radiant  a  is  easily 
determined — it  is  such  that,  calling  the  projections  of 
the  points  E  and  C  upon  the  plane  of  illumination  E1 
and  C1,  a  C1  =  a  E1  ^2 ,  approximately.  To  fulfill  this 
condition  Aa:=Bb  =  i5f  very  nearly,  and  the  two 
radiants  are  at  once  located.  In  this  case  d~  =  994, 
and  since  C  =  L  d2,  C  should  be  practically  1000  candle- 
power.  Allowing  ( — ITT)  =  T'^'  eacn  °f  *ne  radiants 

should  be  of  about  666  candle-power,  a  requirement 
which  could  be  practically  met  by  a  nominal  2OOO-cp 
open  arc,  if  its  glare  were  not  so  forbidding. 

Using  incandescents,  42  of  16  candle-power  would  be 
required  in  each  group,  which  should  be  increased  to 
about  60  if  ground  bulbs  in  a  chandelier  were  to  be 
used,  since  lamps  so  mounted  interfere  with  each  other's 
effectiveness  to  a  certain  extent.  Reducing  these 
figures  to  square  feet  per  candle-power,  it  appears  that 
the  assumed  conditions  are  satisfied  by  allowing  as  a 
maximum  about  3.75  sq.  ft.  per  candle-power,  or  with 
allowance  for  properly  softening  the  light,  2.6  sq.  ft. 
per  candle-power. 

Lighting  such  a  space  from  two  points  only  is  usually 
by  no  means  the  best  way,  and  a  much  better  effect 
would  be  secured  by  using  six  radiants.  The  same  rea- 
soning which  led  us  to  place  a  and  b  near  the  ends  of  the 
major  axis  of  the  room  indicates  a  similar  shifting  in  the 
case  of  six  lights.  From  symmetry,  two  should  be  on 
the  minor  axis  DOC,  and  as  regards  the  projections  of 
C  and  0  on  the  plane  of  illumination,  the  best  position 
for  a  radiant,  located  in  the  same  horizontal  plane  as 


LIGHTING    LARGE    INTERIORS.  215 

before,  is  at  a1,  about  6'  from  0,  with  fr1  at  a  correspond- 
ing point  on  the  other  side  of  0.  Now  for  the  lateral 
pairs  of  lights.  One  of  them  may  be  approximately 
located  with  reference  to  E1,  and  the  projection  of  the 
middle  point  of  the  line  to  a1,  much  as  a1  itself  was 
located.  This  leads  to  a  position  c1,  4.1'  from  a1  and  9' 
from  the  wall.  Forming  now  the  equation 

C  =  ——^ -,  <F  =  306,  ^  =  1906, 


and  the  sum  of  the  other  terms  is  little  greater  than  the 
term  in  d*.  Simplifying  thus,  the  candle-power  of  each 
radiant  comes  out  very  nearly  235,  without  allowance 
for  diffusion  on  the  one  hand  or  for  ground  bulbs  and 
incidental  losses  on  the  other. 

Setting  these  off  against  each  other,  it  appears  that 
the  conditions  call  for  15,  i6-cp  lamps  in  each  of  the  six 
groups,  a  total  of  90  as  against  120  in  the  previous  ar- 
rangement. The  total  rated  candle-power  is  then  1440, 
or  i  candle-power  for  every  3.5  sq.  ft. 

It  is  interesting  to  check  this  computation,  based  en- 
tirely on  an  assumed  minimum  illumination  of  i  candle- 
foot,  with  the  result  of  experiment.  For  large  rooms, 
ranging  from  about  1000  to  5000  sq.  ft.  in  area,  Uppen- 
born's  careful  investigations  show  that  for  good  illumi- 
nation 3  to  3.5  sq.  ft.  per  candle-power  is  the  amount 
required  in  practice.  In  most  cases  these  large  spaces 
are  finished  in  light  color,  so  that  in  spite  of  the  high 
ceilings  they  are  scarcely  more  difficult  to  light  than 
ordinary  dwellings.  The  absolute  brilliancy  required  is 
determined  by  the  purpose  of  the  illumination,  and  the 
proper  arrangement  of  the  lights  depends  largely  on 


2i6  THE   ART   OF   ILLUMINATION. 

architectural  considerations.  Oftentimes  frieze  and 
ceiling  lights  are  used  in  halls,  and  their  application  to 
the  case  in  hand  is  worth  considering. 

If  arranged  as  a  frieze,  the  lamps  would  be  equally 
spaced  around  the  walls,  at  about  5  ft.  below  the  ceiling, 
bringing  them  22  ft.  above  the  plane  of  illumination. 
For  simplicity  we  will  assume  the  use  of  90  i6-cp  re- 
flector lamps,  or  their  equivalent.  Each  gives  approxi- 
mately 27  candle-power  in  its  hemisphere  of  illumina- 
tion. These  lamps  would  be  spaced  a  little  more  than 
3  ft.  apart,  giving  30  on  each  side  of  the  hall  and  1 5  on 
each  end.  Now,  taking  for  examination  the  corner  El, 
which  is  as  unfavorable  a  locality  as  any,  and  roughly 
running  up  the  illumination  at  this  point,  it  falls  a 
little  short  of  i  candle-foot,  but  a  diffusion  factor  of  1.25 
would  carry  it  just  about  to  the  required  amount.  With 
lightly  ground  bulbs,  which  are  far  preferable  to  the 
clear  ones  in  such  a  case,  an  increase  to  36  lamps  on 
each  side  and  18  o<n  each  end  would  be  desirable,  and  40 
and  20  on  sides  and  ends  respectively  would  do  still 
better. 

With  the  original  90  lamps  the  total  rated  hemi- 
spherical candle-power  would  be  2430,  which  is  at  the 
rate  of  2.06  sq.  ft.  per  candle-power. 

Lighting  from  the  ceiling  would  lead  to  a  slightly 
worse  result,  and  it  is  safe  to  say  that  an  increase  of  30 
to  50  per  cent,  in  the  total  candle-power  of  the  radiants 
is  required  in  changing  to  frieze  or  ceiling  lighting  from 
pendent  or  side  lights.  Lights  so  arranged,  however, 
can  give  a  very  valuable  groundwork  of  illumination 
when  re-enforced  by  lights  more  favorably  placed. 
They  have  the  advantage  of  being  unobtrusive  and  of 
producing  a  generally  brilliant  effect,  but  give,  if  rsed 


LIGHTING    LARGE    INTERIORS.  217 

to  the  exclusion  of  everything  else,  an  illumination  pain- 
fully lacking  in  chiar-oscuro,  and  light  directed  almost 
entirely  downwards  is,  moreover,  somewhat  trying,  like 
a  stage  scene  in  the  absence  of  footlights. 

As  has  been  already  explained,  the  illumination  at  any 
particular  point  should  have  a  predominant  direction, 
else  the  effect  on  the  eyes  is  apt  to  be  serious,  A  room 
lighted  by  brilliantly  phosphorescent  wall  paper,  for  ex- 
ample, would  produce  a  most  disagreeable  effect  unless 
the  luminosity  were  confined  to  one  side,  or,  in  general, 
to  limited  portions  of  wall. 

Something  of  the  same  objection  appertains  to  ceil- 
ing or  frieze  lighting  when  pushed  to  an  extreme.  In 
the  room  under  discussion,  the  best  general  effect  would 
probably  be  produced  by  combining  pendent  or  brack- 
eted lights  with  about  an  equal  amount  of  illumination 
from  frieze  or  ceiling  lights.  If  the  room  were  to  be 
used  for  purposes  like  manufacturing,  lighting  from 
rather  powerful  incandescents,  in  part  with  reflectors, 
placed  at  a  convenient  height  above  the  machines,  would 
be  the  most  efficient  procedure. 

Where  merely  rough  work  is  being  done,  arcs  may  be 
effectively  used,  always,  however,  shaded  by  ground  or 
similar  globes.  These  are  distinctly  cheaper,  because 
more  efficient,  than  incandescents,  but  their  light  lacks 
the  steadiness  desirable  for  work  requiring  close  atten- 
tion. Six  35O-watt  arcs  would  give,  in  the  room  shown 
in  Fig.  89,  very  good  illumination  when  placed  in  ap- 
proximately the  positions  deduced  for  the  six  chande- 
liers, with  a  total  expenditure  of  2100  watts  as  against 
about  4500  watts  required  by  the  clustered  incandes- 
cents, and,  say,  3600  watts  required  by  about  36  pendent 
32-cp  lamps.  In  many  cases  less  light  than  this  would 


2i8  THE   ART    OF   ILLUMINATION. 

be  required,  and  the  total  amount  of  energy  could  be 
correspondingly  reduced,  but  about  the  above  ratios 
would  hold  good. 

From  Fig.  89  it  appears  that  in  using  arcs,  about  2000 
to  2500  sq.  ft.  may  be  assigned  to  each  5OO-watt  arc,  and 
1000  to  1500  sq.  ft  to  each  35O-watt  arc.  It  should  be 
remembered  that  the  enclosed  arcs  with  inner  globes 
are  somewhat  less  efficient  than  this,  although  greatly  to 
be  preferred  by  reason  of  their  steadiness,  and  that  alter- 
nating arcs  are  slightly  less  efficient  than  continuous- 
current  arcs. 

Arcs  do  their  best  work  when  placed  fairly  high  and 
used  in  cases  where  protracted  close  attention  on  the 
part  of  the  workmen  is  not  necessary.  They  are  some- 
what preferable  to  incandescents,  too,  when  colored  ob- 
jects are  to  be  illuminated. 

In  workshops  where  special  objects  are  to  be  illumi- 
nated, arcs  are  at  a  great  disadvantage  with  respect  to 
the  distribution  of  light,  since  their  relatively  small 
number  forbids  placing  them  in  the  most  advantageous 
positions  with  respect  to  all  the  machines.  They  have, 
in  short,  the  disadvantage  of  being  radiants  too  power- 
ful for  the  best  distribution.  It  is  thus  found  that  in 
practical  illumination  arcs  are  considerably  less  efficient 
than  their  actual  candle-power  would  indicate.  The 
effect  of  the  bright  radiant  upon  the  eyes,  the  rather 
dense  shadows  and  the  slanting  light  at  a  distance  from 
the  arc,  unite  to  produce  results  that  cannot  be  predi- 
cated from  photometric  measurements  alone. 

For  example,  a  35O-watt  open  arc  is,  in  point  of  mean 
spherical  candle-power,  closely  equivalent  to  ten  32-cp 
incandescent  lamps;  but  in  an  actual  installation  in-doors 
there  are  few  cases  in  which  the  arc  could  not  be  replaced 


LIGHTING    LARGE    INTERIORS.  219 

by  six  such  incandescents  without  detriment  to  the  illumi- 
nation. These  interesting  questions  will  be  the  object 
of  some  future  attention,  but  the  obvious  continuation 
of  the  present  problem  is  the  adaptation  of  gas  lighting 
to  the  case  in  hand. 

If  mere  illumination  is  the  object  to  be  attained,  there 
is  little  doubt  that  mantle  burners  should  invariably  be 
used  in  rooms  of  the  size  considered.  As  already  inti- 
mated, each  such  burner  of  the  ordinary  size  is  equiva- 
lent to  about  two  i6-cp  distributed  incandescents.  If 
the  lamps  are  grouped  in  each  case,  the  mantle  burner 
must  be  given  a  rather  better  rating,  being  equivalent 
to  between  2.5  and  3  such  incandescents.  Properly 
shaded,  the  mantle  burner  is  a  very  economical  and 
effective  illuminant.  Were  it  not  for  the  very  objection- 
able color  of  the  unshaded  light,  it  would  be  much  more 
extensively  used  than  it  is  at  present. 

For  lighting  large  areas,  like  the  one  we  have  been 
considering,  it  is  very  well  adapted,  but  if  the  lights  are 
placed  high  it  is  necessary  not  only  so  to  shade  them  as 
to  correct  the  color,  but  they  must  in  addition  be  fur- 
nished with  such  shades  or  reflectors  as  will  throw  the 
light  downward ;  for  it  must  be  remembered  that  mantle 
burners  must  be  placed  with  the  mantle  in  a  substan- 
tially vertical  position,  and  give  the  maximum  intensity 
of  light  a  little  above  rather  than  below  the  horizontal 
plane,  while  incandescent  lamps,  which  we  have  been 
chiefly  considering,  throw  the  light  in  more  nearly  a 
spherical  distribution,  although  really  considerably  de- 
parting from  it.  Reflectors  or  holophane  globes  used 
with  the  mantle  burners  will  correct  this  faulty  distribu- 
tion and  enable  them  to  be  used  more  effectively  in  the  case 
in  hand. 


220  THE   ART   OF   ILLUMINATION. 

In  rooms  lower  than  that  already  considered  it  is  de- 
sirable to  increase  the  number  of  radiants  considerably, 
to  avoid  too  oblique  illumination  at  the  more  distant 
part  of  the  field  of  each  light. 

With  higher  rooms,  on  the  contrary,  one  can  concen- 


4 


FIG.  90. — Vertical  Section  of  Hall. 

trate  the  radiants  more  advantageously,  and  has  con- 
siderable more  liberty  of  action  in  placing  the  lights. 

Fig.  90  is  intended  to  illustrate  the  conditions  which 
exist  in  a  very  high  room  of  fairly  large  area.  It  shows 
in  vertical  section  a  room  supposed  to  be  50  ft.  square 
and  50  ft.  high,  the  plane  of  illumination,  a  b,  being  3  ft. 
from  the  floor.  We  have  here  2500  sq.  ft.  of  floor 
surface.  At  the  ordinary  rate  of  3  sq.  ft.  per  candle, 


LIGHTING    LARGE    INTERIORS.  221 

this  would  demand  833  candle-power,  or  practically  52 
i6-cp  lamps,  or,  with  a  coefficient  of  diffusion  of  1.50, 
about  36  such  lamps. 

But  the  previous  calculations  having  been  made  for  a 
room  only  one-half  this  height,  and  with  lamps  placed 
considerably  below  the  ceiling,  it  is  clear  that  the  greatly 
increased  height  in  the  present  case  will  lead  to  some- 
what different  conditions  unless  the  lamps  are  to  be 
dropped  very  far  below  the  ceiling — so  low  as  to  pro- 
duce a  decidedly  unpleasing  effect.  Lamps  placed,  for 
example,  in  the  plane  c  d,  corresponding  to  frieze  lamps 
in  the  previous  instance,  are  too  low  to  look  well,  while 
they  would,  on  the  basis  just  given,  furnish  the  room 
with  satisfactory  illumination.  If  placed  on  side 
brackets  at  or  below  the  plane  c  d,  they  would  work  well 
on  the  floor,  but  would  produce  the  effect  of  the  ceiling 
fading  into  dimness  unless  the  ceiling  itself  had  an  ex- 
tremely light  finish. 

Such  a  room,  therefore,  while  very  easy  to  light 
thoroughly,  is  very  difficult  to  light  both  thoroughly  and 
with  good  artistic  results.  Rooms  of  such  dimensions 
are  seldom  used  for  manufacturing  purposes,  these 
shapes  occurring  more  frequently  in  rooms  for  public 
uses  of  various  kinds. 

Witho-ut  going  into  detailed  computation  which  the 
reader  can  readily  make  for  himself  in  the  light  of  pre- 
vious work  on  Fig.  89,  it  is  safe  to  say  that  by  far  the 
best  general  effects  would  be  produced  by  placing  per- 
haps one-third  of  the  total  candle-power  in  8-cp  reflector 
lamps  as  a  frieze,  8  or  10  ft.  below  the  ceiling,  in  the  line 
e,  f,  or  thereabouts,  and  putting  the  remainder  on  brack- 
ets, in  groups  of  three  to  six,  a  little  below  the  plane  c  d. 
Such  an  arrangement  obviously  loses  somewhat  in  the 


222  THE   ART   OF   ILLUMINATION. 

efficient  disposition  of  light,  on  account  of  the  great 
height  of  the  lamps  in  the  frieze,  which  can  be  depended 
on  only  for  a  rather  faint  groundwork  of  illumination  on 
the  plane  of  illumination  a,  b.  If,  for  example,  the  total 
installation  consists  of  600  candle-po<wer,  of  which  200 
is  in  the  frieze,  the  mean  distance  of  the  frieze  lamps 
from  a  point,  say,  in  the  middle  of  the  floor,  would  be  in 
the  vicinity  of  45  ft. 

Consequently,  allowing  for  the  effect  of  the  reflectors 
of  the  frieze  lamps,  and  for  what  each  can  do  by  diffu- 
sion, it  is  safe  to  say  that  the  frieze  lamps  would  give  an 
illumination  of  not  over  one-fifth  candle-foot  on  the 
plane  of  illumination.  Hence,  something  like  eight- 
tenths  candle-foot  would  have  to  be  furnished  by  the 
lights  upon  brackets.  The  amount  of  light  furnished 
by  these  would  therefore  have  to  be  about  eight-tenths 
of  the  total  illumination,  as  determined  by  lights  placed 
in  the  relative  position  shown  by  Fig.  89,  that  is,  the 
ceiling  lights  of  one-third  the  total  candle-power  really 
would  be  furnishing  not  over  one-fifth  of  the  total  light, 
which  means  that  for  lights  placed  as  just  indicated, 
the  total  candle-power  installed  should  be  increased 
somewhere  from  25  to  33  per  cent.,  or  rather  more,  as 
the  bracket  lights  cannot  be  conveniently  placed  in  fav- 
orable situations. 

Hence  in  a  room  so  illuminated  it  would  not  be  safe 
to  allow  more  than  2  to  2^2  sq.  ft.  of  floor  space  per 
candle-power,  and  generally  nearer  the  former  figure 
than  the  latter.  To  attempt  the  lighting  of  such  a  room 
by  frieze  or  ceiling  lights,  as  ordinarily  placed,  would  be 
wasteful.  If  economy  is  not  an  important  factor  in  de- 
signing the  illumination,  at  least  half  the  lights  may  be 
placed  in  the  frieze  with  a  distinct  gain  in  artistic  effect. 


LIGHTING    LARGE    INTERIORS.  223 

In  such  case  the  total  installation  should  be  fully  50  per 
cent,  greater  than  the  minimum  required.  We  shall 
see,  however,  that  there  are  effective  methods  of  getting 
a  strong  groundwork  illumination  from  above  without 
resorting  to  either  of  these  methods. 

To  follow  up  the  effect  of  raising  the  lights  in  a  high 
room  still  further,  it  is  well  to  note  that  the  critical  point 
is  the  amount  of  available  diffusion.  If  one  were  deal- 
ing with  a  room  lined  with  black  velvet,  or  with  translu- 
cent walls,  in  which  there  is  only  a  very  minute  amount 
of  diffused  light,  raising  the  lights  would  diminish  the 
illumination  almost  exactly  according  to  the  law  of  in- 
verse squares. 

Writing  now  K  for  the  coefficient  of  diffusion  denoted 

by  the  fraction  (     _  ^  I ,  and  recurring  to  the  formulae 

previously  given  for  illumination,  we  have  at  once  K  C 
=  L  d2,  and  for  fixed  values  of  C  and  L,  d  =  P  ^/  K, 
where  P  is  a  constant.  Hence  we  may  conclude  that 
for  any  desired  value  of  the  illumination  with  a  fixed 
amount  of  lights  available,  the  height  to  which  these 
lights  can  be  raised  and  still  produce  the  required  effect 
is  approximately  proportional  to  the  square  root  of  the  co- 
efficient of  diffusion. 

The  moral  of  this  is  tolerably  obvious.  If  one  deals 
with  a  dome  finished,  let  us  say,  in  white  and  gold,  it 
may  be  permissible  to  place  a  large  part  of  the  lights 
fairly  high  up,  while  in  a  church  with  a  vaulted  roof  in 
dark  oak,  lights  placed  high  are  nearly  useless  for  pur- 
poses of  illumination.  In  such  a  case  lights  placed  at 
the  level  of  the  roof  beams  and  unprovided  with  re- 
flectors have  barely  more  than  a  decorative  value,  and 
should  be  treated  essentially  as  a  decorative  feature,  use- 


224  THE   ART   OF   ILLUMINATION. 

ful  for  bringing  out  the  details  of  the  architectural 
design. 

Any  real  illumination  must  be  accomplished  by  lamps 
with  reflectors  or  by  lamps  placed  down  nearer  the  plane 
of  illumination.  In  these  dark  interiors  reflector  lamps 
can  be  used  to  especial  advantage,  since  the  coefficient 
of  diffusion  is  so  small  that  the  lessened  diffusion  due  to 
the  partially  directed  beams  from  reflector  lamps  is  of 
trivial  consequence.  In  fact,  there  are  few  cases  in 
which  reflectors  cannot  be  used  to  advantage  in  rooms 
having  very  high  roofs. 

Churches  are  generally  badly  lighted,  and  are,  in  fact, 
rather  difficult  of  treatment,  if  of  any  considerable  size. 
They  are  seldom  brilliant  in  interior  finish,  usually  have 
rather  high  vaulted  roofs,  and  require  good  reading  illu- 
mination. The  few  cases  in  which  their  form  approxi- 
mates to  Fig.  89  may  easily  be  treated  as  there  indicated, 
but  such  is  not  the  usual  condition.  Fig.  QI  gives  a 
roughly  typical  church  floor  plan  as  regards  the  main  body 
of  the  building.  The  total  floor  space  is  shown  as  5000 
sq.  ft.  in  the  nave  and  choir"  combined,  and  800  sq.  ft.  in 
each  transept.  The  walls  are  assumed  to  be  30  ft.  high  in 
the  clear,  with  a  Gothic  roof  above.  Now  the  total  area  to 
be  lighted  is  6600  sq.  ft.,  and  the  value  of  K  is  low,  not 
safely  to  be  taken  as  exceeding  1.20.  The  peculiarities  of 
the  building,  as  a  problem  in  lighting,  lie  in  the  high  walls 
and  the  absence  of  any  ceiling,  both  of  which  complicate 
matters. 

As  to  the  nature  of  the  radiants,  when  electric  lights 
are  available,  one  must  depend  almost  entirely  upon  in- 
candescents.  Arc  lamps  are  not  to  be  considered  for 
artistic  reasons,  save  perhaps  in  indirect  lighting  of  the 
choir.  If  only  gas  is  available,  mantle  burners  suitably 


LIGHTING    LARGE    INTERIORS. 


225 


and  thoroughly  shaded  had  better  be  the  main  reliance, 
as  ordinary  gas  flames  are  seldom  steady  in  such  a  place. 
In  either  case  avoid  chandeliers  as  you  would  shun 
poison.  A  huge  circle  of  lights  pendent  from  a  Gothic 


Fig.  91. 

roof  is  about  as  bad  technically  and  artistically  as  any- 
thing that  could  be  imagined. 

As  to  the  amount  of  light  needed,  it  would  be  ad- 
visable to  allow  no  more  than  2.5  sq.  ft.  per  candle- 
power,  which,  taking  K  at  1.20,  would  call  for  2200  net 


226  THE   ART    OF   ILLUMINATION. 

candle-power.  In  point  of  fact,  in  using  electricity,  not 
less  than  150  i6-cp  lamps  should  be  used,  and  even  this 
number,  on  account  of  the  trying  conditions,  would 
have  to  be  very  deftly  arranged  to  give  the  required  re- 
sult. For  the  best  effect  they  should  be  chiefly  reflector 
lamps,  assigned  about  as  follows:  90  to  the  nave,  20  to 
the  choir,  and  20  to  each  transept.  As  to  position,  the 
most  efficient  method  would  be  to  put  them  in  groups 
of  six  or  eight  on  brackets  between  the  windows,  at  half 
to  two-thirds  the  height  of  the  wall,  with  possibly  larger 
groups  massed  at  the  four  corners  of  the  crossing. 
With  still  more  lights  available  very  beautiful  results 
could  be  attained  by  adding  lights  at  the  capitals,  and, 
in  some  cases,  along  the  tie-beams,  or  on  the  brackets 
from  which  the  pendent-posts  rise.  These  latter  ar- 
rangements are  very  effective,  but  not  economical,  and 
if  used  should  be  installed  on  the  basis  of  about  i  candle- 
power  per  2  sq.  ft.  of  floor  surface.  All  incandescent 
lamps  used  without  diffusing  shades  should  have  ground 
bulbs. 

In  lighting  with  gas,  brackets  are  about  the  only 
thing  feasible,  since  the  flames  must  point  upward,  and 
few  capitals  would  fail  to  look  overloaded  with  ade- 
quately shaded  burners.  Mantle  burners,  of  course,  do  the 
work  most  efficiently,  but  used  alone  the  effect  is  certain 
to  be  grimly  utilitarian,  and  especially  around  the  choir 
small  ordinary  jets  may  be  used  to  very  great  advantage. 
The  mantle  burners  should  be  as  unobtrusive  as  pos- 
sible in  such  a  case,  even  if  they  do  the  main  work  of 
the  illumination. 

Only  the  barest  hints  can  be  given  for  the  detail  of 
church  lighting,  as  so  much  depends  on  the  archi- 
tectural peculiarities  and  on  the  scheme  of  decoration, 


LIGHTING    LARGE    INTERIORS.  227 

but  the  foregoing  indicates  the  general  principles  to  be 
followed.  The  most  important  thing  is  to  give  a  rather 
brilliant  illumination  without  the  individual  radiants 
obtruding  themselves  unpleasantly  on  the  eyes  of  the 
congregation. 

Large  public  buildings  are  generally  easier  to  light 
than  churches,  since  they  are,  as  regards  the  shape  of 
the  several  rooms,  comparatively  simple  and  are  seldom 
dark  in  finish.  Many  rooms  may  be  illuminated  along 
the  lines  already  laid  down,  but,  on  the  whole,  powerful 
radiants,  such  as  arc  lights,  may  be  more  freely  used 
here  than  elsewhere,  thereby  effecting  a  very  considerable 
economy. 

In  high  corridors  and  high  halls  without  galleries  arc 
lights  can  be  used  with  very  excellent  results.  They 
should  invariably  be  shielded  by  ground  or  opal  globes, 
and,  if  hung  very  high,  as  is  sometimes  desirable,  to  keep 
them  out  of  the  ordinary  field  of  vision,  should  be  pro- 
vided with  reflectors.  They  should  be  numerous 
enough  to  suppress  the  shadows  that  ordinarily  ,exist 
under  the  lamps.  In  the  absence  of  such  shadows  the 
modern  enclosed  arcs  have  a  very  material  advantage. 

Rooms  lighted  by  arc  lamps  ought  to  be  of  light 
finish,  since  the  lamps  must  be  placed  rather  high  to 
keep  them,  even  shaded,  from  glaring  unpleasantly,  and 
they  give  a  strong  nearly  horizontal  beam  which,  in  lack  of 
good  diffusing  surfaces,  is  for  the  most  part  wasted. 
Reflectors  deep  enough  to  turn  this  downward  would 
usually  be  most  unsightly  and  would  give  an  unpleasant 
searchlight  effect,  which  should  be  avoided. 

Never  let  the  eye  rest  simultaneously  on  arc  and  in- 
candescent lamps  indoors  or  out,  since  the  latter  seem 
very  dim  and  yellowish  in  such  company,  and  will  never 


228  THE   ART    OF   ILLUMINATION. 

be  credited  with  anything-  like  their  real  brilliancy. 
Similar  reasoning  applies  to  the  use  of  mantle  burners 
and  ordinary  gas  jets  in  the  same  room.  When  so  used 
the  former  should  be  well  shaded  and  unobtrusively 
placed,  and  the  latter  massed  and  generally  unshaded  or 
lightly  shaded,  so  as  not  to  seem  of  relatively  very  small 
intrinsic  brilliancy. 

Sometimes  in  large  interiors  the  powerful  regener- 
ative burners  may  find  a  place.  They  give  an  excellent 
downward  illumination,  which  is  occasionally  very 
useful. 

Theaters  present  some  very  interesting  problems  in 
illumination  on  account  of  their  peculiar  shape  and  the 
difficulty  of  lighting  the  interior  with  sufficient  bril- 
liancy without  making  the  radiants  altogether  too  con- 
spicuous. They  are,  as  a  rule,  far  more  brightly  lighted 
than  other  interiors,  but  seldom  judiciously.  The 
usual  fault  is  to  place  the  lights  so  that  they  shine  di- 
rectly in  the  eyes  of  a  considerable  part  of  the  audience. 
The  auditorium  is  commonly  very  high  in  proportion  to 
its  area,  and  plentifully  supplied  with  galleries.  Fig. 
92  shows  the  typical  elevation,  the  floor  plan  being  gen- 
erally only  slightly  oblong.  The  galleries,  of  course, 
sweep  around  the  sides,  narrowing  as  they  near  the 
proscenium  boxes.  Not  infrequently  a  fourth  gallery  is 
added. 

During  the  acts  no  very  considerable  amount  of  light 
is  needed,  but  between  them  it  is  generally  desirable  to 
produce  an  effect  of  great  brilliancy.  The  main  floor  is 
far  below  the  roof,  and  the  shelving  galleries  render  it 
difficult  to  light  the  spaces  between  them.  The  general 
fittings  are  usually  light,  but  the  dull  hue  of  the  floor  and 
galleries  when  occupied  kills  much  of  the  diffusion. 


LIGHTING    LARGE    INTERIORS. 


229 


The  actual  floor  space  to  be  dealt  with  as  a  problem 
in  illumination  includes  the  galleries,  and  hence  greatly 
exceeds  the  area  of  the  main  floor.  Assuming  the 
width  in  Fig.  92  to  be  50  ft.,  the  nominal  area  in  front  of 
the  footlights  is  3000  sq.  ft.  The  total  gallery  area  is 


1 


Fig.  92. — Elevation  of  Theater. 

usually  from  i  to  1.5  times  the  floor  space,  so  that  the 
entire  space  to  be  lighted  would  be  at  least  6000  sq.  ft., 
half  of  it  being  located  so  that  it  can  get  little  advantage 
from  the  illumination  of  the  main  space  above  the  floor. 
The  space  behind  A,  and  the  galleries  B,  C,  and  to  a  less 
extent  D,  have  to  be  treated  almost  as  separate  rooms, 


23o  THE   ART   OF   ILLUMINATION. 

particularly  when,  as  sometimes  happens,  the  galleries 
are  rather  lower  than  shown  in  Fig.  92. 

This  is  the  main  reason  for  the  apparently  abnormal 
amount  of  light  that  is  needed  in  theaters.  The  fact  is 
that  there  is  really  a  very  great  area  to  light,  and  it  is 
so  placed  that  it  cannot  readily  be  treated  as  a  whole. 
The  following  table  shows  the  approximate  amount  oi 
illumination  furnished  in  a  number  of  prominent  Conti- 
nental theaters. 

If  in  Fig.  92  we  allow,  on  account  of  the  high  ceiling 
and  conditions  unfavorable  for  diffusion,  2  to  2.5  sq.  ft. 
per  candle-power,  and  take  account  of  the  real  total  floor 
space,  including  the  galleries,  we  reach  just  about  the 
figures  given,  which  are  based  on  the  floor  plan  only. 
And  in  practice  3600  candle-power  would  probably  do 
the  work  well,  although,  since  this  only  allows  ordi- 
nary good  reading  illumination,  more  light  is  neces- 
sary to  give  the  really  brilliant  effect  which  is  usually 
desired.  Nearly  5000  candle-power  would  be  required 
to  show  off  the  house  effectively. 

THEATER.  SQ.  FT.  PER  CP.      CP.  PER  SQ.  FT. 

Opera,  Paris.    0.78  1.28 

Opera,  Paris,  as  ballroom 0.38  2.63 

Odeon,  Paris 1.52  0.66 

Gaiete,  Paris 1.14  0.87 

Palais-Royal,  Paris 0.51  1.96 

Renaissance,  Paris 0.52  1.92 

La  Scala,  Milan 1.07  093 

Massimo,  Palermo  (ordinary) 0.86  1.16 

Massimo,  Palermo  (en  fete)  .. 0.53  1.88 

As  to  the  location  of  the  lights  and  their  character, 
the  body  of  the  house  can  be  usefully  lighted  by  lamps 
ranged  along  the  galleries  at  a  b  c.  If  these  are  placed 
below  the  edges  of  the  galleries  they  will  glare  directly 
into  the  eyes  of  the  spectators,  so  that  it  is  better  to  illu- 
minate the  gallery  spaces  from  the  rear  and  above,  at 


LIGHTING    LARGE    INTERIORS.  231 

a'  br  c.  The  radiants  may  well  be  provided  with  re- 
flectors, as  the  diffusion  amounts  to  little,  and  all  lamps 
on  and  under  the  galleries  should  have  ground  globes. 
These  lights  may  be  re-enforced  to  great  advantage  by 
ceiling  reflector  lamps,  best  sunk  in  the  ceiling  deep 
enough  to  make  them  inoffensive  from  the  galleries. 
These,  with  some  ornamental  lighting  about  the  stage 
and  boxes,  should  give  a  capital  result.  The  main  point 
is  to  light  the  interior  brightly  without  thrusting  bright 
radiants  into  the  field  of  vision. 

A  very  beautiful  example  of  theater  lighting  is  shown 
in  the  frontispiece,  a  photograph  from  the  stage  of  the 
Metropolitan  Opera  House,  decorated  for  the  performance 
in  honor  of  Prince  Henry  of  Prussia.  The  temporary 
festoons  from  the  ceiling  are  highly  decorative,  but  better 
suited  to  temporary  than  to  permanent  use,  since  they 
shine  directly  into  the  eyes  of  the  occupants  of  the  gal- 
leries. In  this  instance  the  curtain  was  brilliantly  studded 
with  temporary  incandescents,  and  the  whole  interior  was 
elaborately  decorated. 

A  useful  form  of  ceiling  lighting,  applicable  to  many 
very  high  interiors,  is  arranged  by  replacing  the  lamps 
at  d,  Fig.  92,  by  opal  glass  skylights  of  rather  large  di- 
mensions, and  placing  above  them  arc  lamps  with  re- 
flectors. The  skylight  surfaces  should  be  flat  or  slightly 
projecting  rather  than  recessed,  and  the  reflectors 
should  be  planned  so  that  each  may  throw  a  cone  of 
light  subtending  an  angle  equivalent  to  the  whole  floor 
plan. 

By  thus  superposing  the  indirect  illumination  from  a 
group  of  lamps  the  general  steadiness  of  the  light  is 
greatly  increased.  In  thus  using  arcs  care  should  be 
taken  to  have  the  diffusing  skylights  faintly  tinted  so  as 


232  THE   ART   OF   ILLUMINATION. 

to  lessen  the  color  contrast  between  the  powerful  ceiling 
lights  and  the  incandescents  used  elsewhere  in  the 
house.  It  is  a  considerable  advantage  thus  to  place 
lights  above  the  ceiling,  as  it  avoids  the  serious  heating 
effect  due  to  massing  incandescents  near  the  ceiling  of 
a  generally  overheated  room. 

On  account  of  this  heating  the  use  of  gas  in  theaters 
is  highly  undesirable,  and  has  been  almost  completely 
abandoned.  In  lack  of  anything  better,  fair  results 
could  be  reached  by  mantle  burners  placed  somewhat  as 
shown  in  Fig.  92,  and  very  thoroughly  shaded  by  holo- 
phane  or  other  diffusing  globes,  much  of  the  illumination 
being  located  above  the  ceiling  in  the  form  of  mantle  or 
regenerative  burners. 

Large  and  high  spaces  cannot  often  be  lighted  very 
efficiently,  as  the  conditions  ordinarily  preclude  placing 
powerful  radiants  near  the  plane  of  illumination.  The 
natural  riposte  is  to  use  highly  efficient  radiants,  and  with 
them  to  employ  reflectors  freely.  Hence  the  form  of 
ceiling  illumination  just  explained. 

In  very  large  interiors  without  high  galleries,  arc 
lighting  may  be  very  effectively  used,  provided  the  arcs 
are  well  shaded.  It  is  wise  to  group  them  so  that  no 
single  arc  shall  entirely  dominate  the  illumination  at 
any  particular  point.  It  is  better  to  lose  a  little  in  uni- 
formity of  the  total  illumination  throughout  the  area 
than  to  take  the  chances  of  flickering,  which  is  not  en- 
tirely suppressed  even  in  the  best  arc  lamps. 

In  a  big  space  arcs  can  be  treated  much  like  incandes- 
cents in  a  small  space,  but  the  detail  of  the  work  varies 
so  much  that  only  very  general  suggestions  can  be 
given.  Often  temporary  illumination  has  to  be  under- 
taken, and  must  be  fitted  to  the  case  in  hand.  One  of 


LIGHTING    LARGE    INTERIORS.  233 

the  most  beautiful  examples  of  such  work  that  ever  fell 
under  the  author's  notice  was  the  illumination  of  Madi- 
son Square  Garden  for  a  chrysanthemum  and  orchid 
show  a  few  years  since.  The  feature  of  this  work  was 
the  very  extensive  use  of  both  arc  and  incandescent 
lamps  enclosed  in  Chinese  lanterns.  The  huge  lanterns 
containing  the  arcs  were  very  striking,  and  the  whole 
effect  was  most  harmonious,  while  the  illumination  was 
thoroughly  good.  It  is  mentioned  here  merely  as  a 
clever  bit  of  temporary  lighting  treated  to  suit  the  par- 
ticular occasion. 

In  this  lighting  of  large  interiors  the  smaller  arcs 
worked  on  constant  potential  circuits  are  very  useful, 
although  not  very  efficient.  Those  taking  5  to  6  am- 
peres give  excellent  service,  and  fair  results  can  be 
obtained  with  lamps  working  down  even  to  4  amperes. 
Such  arcs  are  equivalent  to  from  10  to  15  i6-cp  lamps  in 
practical  effect,  and  give  a  greater  candle-power  per 
watt. 

Incandescent  lamps  of  the  Nernst  type,  if  reduced  to 
a  practical  form,  may  be  utilized  in  a  similar  way,  in 
forming  a  good  basis  of  illumination  where  the  total 
amount  of  light  is  considerable.  In  other  words,  when 
one  is  dealing  with  very  large  enclosed  spaces  the  light- 
ing is  simplified  and  made  more  efficient  by  utilizing  the 
more  powerful  radiants. 

Large  incandescent  lamps  giving  from  50  to  100 
candle-power,  or  even  more,  would  be  very  valuable  but 
for  their  high  cost  and  generally  rather  inadequate  life 
and  efficiency.  Those  of  50  candle-power  are  pretty 
satisfactory,  in  spite  of  their  share  of  these  drawbacks, 
but  the  larger  sizes  are  much  less  advantageous. 

In   certain   cases,   particularly    railway    stations   and 


234  THE   ART   OF   ILLUMINATION. 

other  buildings  likely  to  be  rather  smoky,  arcs  have  to 
be  the  main  reliance,  since  the  globes  of  incandescents 
grow  dim  so  quickly  that  cleaning  them  is  an  almost 
interminable  job.  Hence  it  is  best  to  use  compara- 
tively few  powerful  radiants.  The  arcs  should  be 
carried  rather  high,  say  20  to  25  ft.  above  the  ground  or 
floor.  Assuming  0.5  candle-foot  as  the  minimum,  and 
taking  into  account  the  illumination  due  to  adjacent 
lamps,  each  arc  can  be  counted  on  «to  illuminate  over  a 
distance  at  which  it  gives  0.25  candle-foot.  For  close 
detail  reference  must  be  made  to  the  actual  illumination 
curves  of  the  type  of  lamp  used,  and  the  general  prob- 
lem is  analogous  to  street  lighting,  but  for  lamps  25  ft. 
above  the  floor  the  approximate  distance  between  arcs 
of  the  commoner  kinds,  to  give  the  required  illumina- 
tion, may  be  derived  from  the  following  table: 

APPROXIMATE      DISTANCE 

KIND  OF  ARC.  WATTS  BETWEEN        SQ.  FT. 

PER  ARC.  ARCS.  PER  ARC. 

Direct  current,  open,  6.6  amperes 330  80  6,400 

Direct  current,  open,  9.6  amperes 480  105  11,025 

D.  C.  enclosed,  6.6  amperes 480  90  8,100 

Alternating  enclosed,  6.6  amperes 425  75  5.625 

The  alternating  arc  would  do  relatively  better  if 
placed,  say,  15  ft.  high,  since  its  light  is  thrown  more 
nearly  horizontally,  and  since  all  the  arcs  are  assumed  in 
this  table  to  have  clear  outer  globes,  the  open  arcs,  if 
given  pale  opal  globes,  which  they  should  have  to  lessen 
the  unpleasant  glare,  will  be  about  on  a  par  with  the  en- 
closed arcs.  All  arcs  in  enclosed  places  should  have  at 
least  one  opal  globe,  and  when  used  where,  as  in  railway 
stations,  diffuse  reflection  is  of  small  amount,  should  be 
provided  with  reflectors  to  utilize  the  light  that  would 
otherwise  be  wasted.  This  would  somewhat  improve 


LIGHTING    LARGE    INTERIORS.  235 

the  figures  given  above,  and  it  is  quite  safe  to  say  that 
under  ordinary  circumstances,  with  properly  placed 
arcs,  one  arc  taking  about  6.5  amperes  is  good  for  nearly 
10,000  sq.  ft.  of  floor  space  at  the  assumed  intensity  of 
illumination.  More  lights  are  often  desired  locally  in 
places  where  considerably  more  than  0.50  candle-foot  is 
required,  as  in  the  central  part  of  a  passenger  platform, 
but  they  seldom  would  have  to  be  placed  nearer  than  60 
ft.  apart,  unless  the  traffic  is  exceptionally  great. 

Certain  classes  of  interiors  require,  on  account  of  the 
uses  to  which  they  are  put,  especial  adaptations  of  the 
radiants,  either  in  kind,  amount,  or  position.  One  of  the 
commonest  demands  is  for  an  illumination  of  unusual 
brilliancy  and  steadiness  in  situations  like  reading 
rooms,  draughting  rooms,  schools,  weave  shops,  and 
such  like  places,  where  the  eyes  are  under  steady,  if  not 
severe  strain.  Ordinary  good  reading  illumination, 
such  as  we  have  been  considering,  must  be  considerably 
strengthened  to  meet  these  requirements.  Simple  in- 
crease in  the  number  or  power  of  the  radiants  sometimes 
meets  the  conditions,  if  such  increase  can  be  had  with- 
out thrusting  too  powerful  lights  into  the  field  of  vision. 

It  may  be  necessary  to  furnish  i  candle-power  for  each 
2  sq.  ft.  of  area,  or,  in  extreme  cases,  i  candle-power 
per  square  foot.  One  of  the  most  useful  schemes  for 
supplying  such  large  amounts  of  light  is  the  use  of  the 
inverted  arc  in  connection  with  a  very  light  interior 
finish. 

The  ordinary  continuous-current  arc,  in  virtue  of  the 
brilliant  crater  of  the  positive  carbon,  throws  its  light 
downward;  but  if  the  current  be  reversed  so  as  to  form 
the  bright  crater  on  the  lower  carbon,  most  of  the  light 
is  thrown  upward  toward  the  ceiling,  and  is  there  dif- 


236  THE   ART   OF   ILLUMINATION. 

fused.  If,  as  usual,  these  arcs  are  arranged  with  in- 
verted conical  reflectors  of  enameled  tin  or  the  like,  all 
the  direct  rays  are  cut  off  and  the  entire  illumination  is 
by  the  diffused  rays.  The  result  is  a  very  soft  and  uni- 
form light,  white  in  color,  and  of  any  required  brilliancy. 
Fig.  93  shows  in  diagram  the  principle  of  this  device. 
In  case  a  white  ceiling  is  not  available,  large  white  dif- 
fusing screens  over  the  lamps,  of  enameled  tin  or  even 
of  tightly  stretched  white  cloth  or  paper,  answer  the  pur- 
pose. Indeed,  this  was  the  original  form  of  the  device 
as  shown  by  Jaspar  at  the  Paris  Exposition  of  1881. 

With  reference  to  Fig.  93,  it  is  sufficient  to  note  that 
the  conical  reflector  should  be  rather  shallow,  just  deep 
enough  to  throw  the  light  wholly  on  the  ceiling  and 
upper  walls,  but  shallow  enough  for  two  neighbor- 
ing lights,  as  shown,  to  distribute  light  over  each 
other's  fields,  which  improves  the  average  steadiness  of 
the  illumination.  The  arcs  need  no  diffusing  globes,  a 
clear  globe  being  sufficient,  and  open  arcs  may  be  freely 
used,  to  the  material  improvement  of  the  luminous  effi- 
ciency, never  very  high  in  this  form  of  lighting. 

The  heights  of  the  arcs  should  depend  somewhat  on 
circumstances  regarding  the  appearance  and  the  purpose 
of  the  lights,  but  will  generally  be  half  to  three- fourths  the 
height  of  the  room.  The  reflectors  may  be  from  3  ft. 
to  6  ft.  in  diameter,  and  may  have  an  angle  at  the  apex 
of  1 20  degrees  to  140  degrees.  Only  in  case  of  having 
to  throw  the  light  on  special  screens  rather  than  on  the 
natural  ceiling  should  the  reflectors  have  less  aperture 
than  just  indicated.  They  then  become  of  the  nature  of 
projectors,  and  the  angle  at  the  apex  may  be  90  degrees 
or  so. 

As  to  the  efficiency  of  such  illumination,   one  may 


LIGHTING    LARGE    INTERIORS.  237 

roughly  assume  i  watt  per  spherical  candle-power  for 
powerful  open  continuous-current  arcs,  and  may  reckon 
on  a  loss  of  about  one-half  in  the  process  of  diffuse  re- 
flection. The  diffuse  illumination  may  then  be  taken  as 
being  in  candle-power  about  0.5  to  0.6  the  number  of 
watts  expended,  not  including  artificial  resistance. 
Thus,  a  continuous-current  arc,  taking  9  amperes  to  10 
amperes  at  about  50  volts,  utilized  in  this  manner  will 


Fig.  93. — Lighting  by  Inverted  Arcs. 

illuminate  250  sq.  ft.  to  300  sq.  ft.  on  the  basis  of  i  sq.  ft. 
per  candle-power,  or  500  sq.  ft.  to  600  sq.  ft.  at  2  sq.  ft. 
per  candle-power. 

It  must  be  noted  that  if  enclosed  arcs  are  used  in  this 
way,  materially  less  light  is  obtained,  as  is  well  known. 
Even  with  both  outer  and  inner  globes  clear  one  cannot 
count  on  much  better  than  2  watts  per  mean  spherical 
candle-power,  although  occasional  results  of  1.5  watts 
or  a  little  below  are  attained.  Alternating  lamps  re- 
quire, of  course,  still  more  energy,  and  with  enclosed 


238  THE   ART   OF   ILLUMINATION. 

arcs  in  general  one  would  hardly  find  it  advisable  to 
allow,  when  using  ceiling  diffusion,  more  than  half  to 
three-fifths  of  the  area  per  watt  just  indicated  for  open 
arcs.  Enclosed  arcs  have  no  marked  crater,  which 
operates  somewhat  against  their  effectiveness  in  this  class 
of  lighting. 

These  figures  are  necessarily  only  approximate,  but 
while  enclosed  arcs  have  some  conspicuous  virtues,  high 
efficiency  as  respects  mean  spherical  candle-power  is  not 
one  of  them.  In  all  this  lighting  by  diffusion  the  diffus- 
ing surfaces  must  be  kept  clean,  else  there  will  be  much 
loss  of  light.  Under  even  the  best  conditions  one  does 
not  do  very  much  better  than  2  watts  per  candle  power, 
and  lack  of  care  or  bad  engineering  may  easily  trans- 
form this  into  3  or  4  watts  per  candle-power,  which  is  no 
better  efficiency  than  incandescents  would  give. 

The  chief  advantage  of  this  diffused  lighting  is  that  it 
enables  one  to  secure  very  brilliant  illumination  with 
white  light,  without  trying  the  eyes  with  intense 
radiants. 

This  illumination  has,  however,  one  curious  failing,  in 
that  as  ordinarily  installed  it  is  slwdoidess ,  and  the  light 
has  no  determinate  direction.  For  certain  kinds  of 
work  this  is  a  very  trying  peculiarity,  severely  felt  by( 
the  eyes.  It  may  be  remedied  in  various  ways,  of  which 
perhaps  the  simplest  is  the  lateral  displacement  of  the 
lamps  shown  in  Fig.  94. 

This  gives  a  predominant  direction  to  the  light,  some- 
thing akin  to  the  effect  produced  by  a  row  of  windows 
along  the  side  of  the  room,  and  is  probably  as  near  an 
approach  to  artificial  daylight  as  can  be  attained  by 
simple  means.  It  is  not  unlike  in  principle  the  "  arti- 
ficial moon  "  used  in  the  reading  room  at  Columbia 


LIGHTING    LARGE    INTERIORS.  239 

University,  consisting  of  a  great  white  ball  intensely 
illuminated  by  arcs  backed  by  projectors. 

In  using  the  arrangement  of  Fig.  94,  about  the  same 
relative  number  of  arcs  is  required  as  in  Fig.  93,  but 
they  are  placed  in  one  row  instead  of  two.  The  uni- 
lateral effect  could  be  greatly  enhanced  by  a  diffusive 
screen  a  b,  Fig.  94,  running  along  back  of  the  arcs.  Its 


Fig.  94. — Unilateral  Illumination. 

angle  with  the  ceiling  evidently  should  depend  on  the 
shape  of  the  room. 

Unilateral  illumination,  whether  diffused  or  not,  is 
often  desirable  from  a  hygienic  standpoint.  In  many 
cases  well-shaded  arcs  may  replace  the  diffused  lighting 
just  described,  though  such  direct  lighting  is  generally 
rather  less  steady.  But  it  must  be  remembered  that  an 
arc  having  both  inner  and  outer  globes  opalescent  is 
scarcely,  if  at  all,  more  efficient  than  incandescent  lamps, 
assuming  both  to  be  worked  off  constant  potential 
mains;  hence,  unless  the  whiteness  of  the  arc  light  is 


240  THE   ART   OF   ILLUMINATION. 

essential,  incandescents,  being  steadier,  are  very  often 
preferable. 

In  factories  where  colored  fabrics  are  woven,  and  in 
shops  where  they  are  sold,  white  illumination  is  a  mat- 
ter of  great  importance,  and  arcs  are  especially  useful. 
In  the  mills  the  necessary  illumination  depends  largely 
on  the  color  of  the  fabrics.  It  should,  as  a  matter  of 
experience,  range  from  2  sq.  ft.  per  candle-power  to  i 
sq.  ft.  per  candle-power  in  passing  from  white  to  dark 
and  fine  goods.  The  candle-power  noted  here  is  actu- 
ally mean  spherical,  or  hemispherical,  if  reflectors  are 
used,  taken  from  the  real  performance  of  the  arc  well 
shaded.  This  qualification  means  practically  300  sq.  ft. 
to  400  sq.  ft.  for  each  arc  of  450  watts  to  500  watts  in 
the  extreme  case,  and  600  sq.  ft.  to  800  sq.  ft.  for  white 
and  light-colored  goods.  Shops  where  such  goods 
must  be  sold  by  artificial  light  should  be  lighted  on  very 
nearly  the  same  basis.  For  brilliant  illumination,  where 
color  distinctions  must  be  accurately  preserved,  the  arc 
at  the  present  time  stands  pre-eminent,  and  should  gen- 
erally be  used,  although  Nernst  lamgs  and  acetylene 
flames  have  a  similar  advantage.  It  must  be  remembered, 
however,  that  enclosed  arcs  are  distinctly  bluish  unless  the 
current  is  pushed  up  nearly  to  the  limit  of  endurance  of  the 
inner  globes,  and  hence  when  used  in  situations  where 
color  is  important,  should  have  shades  tinted  to  correct 
this  idiosyncrasy.  The  common  opalescent  inner  globe 
is  entirely  insufficient  for  the  purpose. 

Where  arc  lights  are  not  available,  and  it  is  desired  to 
furnish  approximately  white  light,  there  is  difficulty  in 
meeting  the  requirement.  Mantle  gas  burners  with  ex- 
treme care  in  selecting  tinted  shades  to  correct  the  gen- 
erally greenish  cast  of  the  light  may  be  made  to  give 


LIGHTING    LARGE    INTERIORS.  241 

fair  results,  but  are  considerably  inferior  to  arc  lights. 
Incandescent  lamps  fail  to  meet  the  requirement,  and 
perhaps  the  closest  approximation  to  the  arc  in  the  matter 
of  color  is  to  be  found  in  the  acetylene  flame  or  in  the 
Nernst  lamp. 

In  the  lighting  of  workshops  for  various  purposes,  no 
such  brilliant  illumination  as  has  been  mentioned  with 
reference  to  textile  factories  is  required.  The  most 
economical  scheme  of  illumination  is  to  furnish  general 
illumination  in  moderate  amount,  and  to  re-enforce  it,  in 
points  where  brilliant  light  is  needed,  by  extra  lights  at 
these  places.  So  far  as  the  general  illumination  is  con- 
sidered, i  candle-power  to  about  4  sq.  ft.  or  5  sq.  ft.  is 
ample.  The  extra  lights  should  be  put  bearing  as  di- 
rectly as  possible  on  the  work  in  hand,  and  should  fur- 
nish illumination  at  that  work  to  the  extent  of  from  i 
candle-foot  to  2  or  3  candle  feet,  according  to  the  needs 
of  the  work. 

It  should  not  be  forgotten  that  good  illumination  in 
a  workshop  tends  materially  to  increase  the  quantity  and 
improve  the  quality  of  the  work  turned  out. 

In  most  instances  the  color  of  the  light  within  the 
range  of  ordinary  illuminants  is  not  a  matter  of  consider- 
able importance,  but  the  light  must  always  be  reasonably 
steady.  Hence  the  incandescent  lamp  and  the  mantle 
burner  for  gas  are  by  far  the  most  valuable  sources  of 
light  generally  to  be  found.  Ordinary  bat-wing  gas 
burners  are  probably  the  worst  in  point  of  steadiness, 
although  a  badly  adjusted  electric  arc  is  a  close  second. 

Where  very  powerful  radiants  are  desired,  the  large 
regenerative  gas  burners  give  a  very  brilliant  and  steady 
light.  They  throw  out,  however,  a  great  deal  of  heat, 
which  is  sometimes  objectionable,  and  are  less  econom- 


242  THE   ART    OF   ILLUMINATION. 

ical  of  gas  than  the  mantle  burner.  If  the  Nernst  in- 
candescent lamp  is  brought,  as  it  promises  to  be,  into 
commercial  usefulness,  it  will  prove  exceedingly  valua- 
ble for  the  illumination  of  large  enclosed  spaces,  by 
reason  of  its  considerable  power  and  the  whiteness  and 
steadiness  of  its  light.  At  the  present  time  it  is  not  far 
enough  past  the  experimental  stage  to  be  a  serious  com- 
petitor of  other  illuminants. 

A  very  special  branch  of  illumination  is  the  lighting  of 
immense  enclosed  spaces,  such  as  are  found  in  exposi- 
tion buildings.  This  work  is  on  such  a  large  scale  that 
it  almost  partakes  of  the  nature  of  outdoor  lighting,  with 
which  it  is  very  intimately  connected  as  a  practical  prob- 
lem. The  amount  of  light  required  in  single  enclosed 
spaces  of  colossal  dimensions,  like  exposition  halls,  varies 
considerably  according  to  the  practical  use  to  which  the 
space  is  to  be  put.  As  a  rule,  the  most  brilliant  and 
useful  illumination  in  these  large  spaces  is  secured  by 
the  use  of  arc  lights  to  the  exclusion  of  other  illumi- 
nants. In  a  building  covering  one  or  several  acres,  and 
perhaps  100  ft.  or  more  in  height,  incandescents  of 
ordinary  powers  look  lost;  and  if  the  roof  is  not  to 
fade  away  into  darkness,  a  very  large  number  of  lights 
must  be  required  to  bring  it  into  prominence,  placed  so 
high  from  the  floor  as  to  be  of  little  service  for  the  gen- 
eral illumination. 

Moreover,  such  buildings  have  generally  a  very  large 
amount  of  glazed  side  and  roof  space,  furnishing  the 
ordinary  daylight  illumination.  Consequently  the  walls 
and  ceiling  diffuse  very  little  light.  With  arc  lights  the 
power  of  the  individual  radiants  bears  some  respectable 
proportion  to  the  size  of  the  space  to  be  illuminated. 
The  luminous  efficiency  is  increased,  and  by  sufficient 


LIGHTING    LARGE    INTERIORS.  243 

massing  of  lights  with  reflectors,  even  the  highest  halls 
can  be  admirably  lighted.  The  work  can,  of  course,  be 
beautifully  done  with  incandescents  if  enough  are  avail- 
able, but  at  considerably  lessened  economy. 

The  amount  of  light  required  per  square  foot  of  floor 
space  is  very  considerable,  owing  to  the  height  and  bad 
diffusing  properties  of  the  building,  and  for  the  best  re- 
sults i  actual  candle-power  should  be  furnished  for  each 
2  sq.  ft.  to  3  sq.  ft.,  according  to  conditions. 

Incandescent  lamps  have  a  very  high  decorative  value 
in  connection  with  such  work,  but  to  be  used  effectively 
must  be  massed  somewhere  near  the  plane  of  illumina- 
tion, lights  in  and  about  the  roof  being  practically  only 
for  decorative  purposes.  Used  in  sufficient  numbers, 
however,  they  give,  in  virtue  of  their  complete  sub- 
division of  the  illumination,  a  better  artistic  result  than 
can  be  obtained  with  arcs. 

The  subject  of  exposition  illumination  is  so  large  and 
so  special  in  its  character  as  to  be  hardly  appropriate  to 
the  scope  of  the  present  work. 


CHAPTER  XL 

STREET    AND   EXTERIOR   ILLUMINATION. 

A  SPECIAL  and  very  important  department  of  illumina- 
tion has  to  do  with  streets  and  other  outdoor  spaces. 
It  involves  not  a  few  unusual  difficulties,  for  there  is  un- 
limited space  to  deal  with,  as  well  as  an  indefinite  variety 
of  natural  and  artificial  obstructions.  Save  in  narrow 
streets  bordered  by  high  buildings,  one  gains  little  or 
nothing  from  the  diffusion  that  is  so  important  a  factor 
in  interior  lighting,  and  in  many  instances  the  streets  are 
so  thickly  shaded  by  trees  that  the  problem  of  adequate 
lighting  is  very  difficult. 

Light  above  the  horizontal  plane  is  of  comparatively 
little  value,  \vhile  the  ground  should  receive  a  strong  and 
even  illumination.  In  the  comparative  freedom  from  re- 
flecting and  diffusing  surfaces  there  is,  however,  one 
small  gain,  for  this  is  the  case  in  which  the  law  of  inverse 
squares  holds  good.  Of  course,  there  is  some  diffusion 
from  the  street — when  the  ground  is  snow  covered,  a 
considerable  amount — but,  broadly,  one  can  compute 
the  illumination  with  fair  accuracy. 

In  computing  the  illumination,  however,  two  radically 
different  methods  have  come  into  use.  In  one  of  these 
the  radiant  in  street  lighting  is  supposed  to  illuminate 
a  geometrical  plane,  of  which  each  element  receives 
illumination  depending,  not  only  on  its  distance  from  the 
radiant,  but  on  the  obliquity  of  the  rays  which  strike  it. 
The  other  method  assumes  that  in  reckoning  the  illumi- 

244 


EXTERIOR    ILLUMINATION.  245 

x 

nation  at  any  point  the  element  of  surface  to  be  con- 
sidered is  not  in  the  plane  of  the  gro\ind,  but  in  a  plane 
perpendicular  to  the  direction  of  the  ray. 

The  first  method  would  determine  the  legibility  of  a  bit 
of  newspaper  lying  fiat  on  the  pavement,  the  second  method 
the  visibility  of  a  stone  projecting  above  it.  The  distinc- 
tion might  at  first  appear  like  that  between  tweedle-dum 
and  tweedle-dee,  but  in  fact  it  makes  an  absurdly  great 
difference  in  the  theoretical  illumination.  Fig.  95 
shows  the  conditions  which  arise  in  the  two  cases.  As- 
sume a  light  L  of  1000  uniform  spherical  candle-power 
at  the  top  of  a  pole  25  ft.  high,  and  then  calculate  the 


Fig.  95. — Illumination  at  a  Surface. 

illumination  at  a  surface  S,  100  ft.  from  the  foot  of  the 
pole,  by  the  law  of  inverse  squares  on  the  two  hy- 
potheses. In  the  first  place,  taking  account  of  the 
obliquity,  calling  the  illumination  /,  we  have 

L  sin  a  ,,     .. 

/  =     a          g  =  0.023  candle  feet. 

But  considering  S,  the  surface,  as  on  a  projecting  cob- 
blestone at  the  same  point, 

/  =  ,a      ^  —  0.094  candle  feet. 
And  the  worst  of  the  discrepancy  is  that  it  is  greatest  at 


246  THE   ART    OF   ILLUMINATION. 

considerable  distances  from  the  radiant,  where  the  light 
is  feeble  at  best. 

Now  in  point  of  fact,  the  object  of  lighting  streets  is 
neither  to  enable  one  to  read  a  page  laid  flat  on  the  pave- 
ment nor  to  observe  a  surface  perpendicular  to  the  ray. 
But  the  intensity  and  distance  of  the  radiants  is  deter- 
mined by  the  minimum  illumination  allowable  midway 
between  lights,  and  not  by  the  brightness  of  the  regions 
near  the  poles,  and  in  these  comparatively  dimly  lighted 
spaces  the  things  which  must  be  observed  and  for  which 
the  users  of  streets  keep  their  eyes  open  are  things  not 
in  the  plane  of  the  pavement,  but  above  it.  For  ob- 
serving these  the  obliquity  of  the  light  as  regards  the 
pavement  does  not  much  matter;  in  fact,  at  equal  inten- 
sities oblique  light  gives  stronger  contrasts  than  per- 
pendicular light. 

Under  these  circumstances,  however,  one  is  justified 
in  reckoning  upon  the  illumination  received  from  one 
direction  only.  A  printed  page  laid  on  the  pavement  at 
the  point  S  receives  useful  light  from  both  the  neighbor- 
ing radiants,  and  so,  if  5*  is  halfway  between  two  lamp- 
posts, it  will  really  receive  on  the  first  hypotheses  0.046 
candle-foot.  But  a  projecting  stone  or  the  face  of  a 
wayfarer  is  illuminated  so  far  as  a  given  viewpoint  is  con- 
cerned by  only  one  of  the  adjacent  radiants. 

For  these  reasons,  in  the  ensuing  discussion,  the  illu- 
mination at  a  point  6"  will  be  assumed  to  be  that  which 
falls  on  either  hemisphere,  let  us  say,  of  a  billiard  ball  at 
S.  It  will  then  be  reckoned  by  the  second  formula, 

/==  #>  +  </» 

but  only  one  radiant,  or  the  radiants  on  one  side  only, 
will  be  considered  as  useful. 


EXTERIOR    ILLUMINATION.  247 

It  must  not  be  understood  from  the  preceding  state- 
ment as  to  the  importance  of  the  minimum  illumination 
that  the  lighting  of  a  street  should  be  specified  in  terms 
of  this  minimum  only.  For  such  a  proceeding  leads  at 
once  to  a  reductio  ad  absurdum.  And  this  is  particularly 
the  case  if  the  illumination  on  the  plane  of  the  pavement 
is  considered.  For  assuming  a  sidewalk  lighted  by 
common  candles  placed  6  ft.  apart  and  6  ft.  high,  and 
reckoning  only  the  first  four  candles  on  each  side  of  a 
point  on  the  pavement,  the  illumination  based  on  the 

formula 

L  sin  a 

~  tf  +  </a 

amounts  to  no  less  than  0.052  candle-foot  as  against 
0.046  candle-foot  from  a  pair  of  powerful  arc  lights 
placed  200  ft.  apart.  In  other  words,  the  street  on  this 
hypothesis  would  be  better  lighted  by  the  candles  giv- 
ing less  than  one-sixtieth  the  actual  amount  of  light.  A 
very  slight  effort  of  the  imagination  will  picture  the  really 
vivid  contrast  between  the  two  conditions. 

In  determining  the  conditions  with  reference  to  the 
minimum  illumination,  therefore,  one  must  also  take  into 
account  the  real  amount  of  light  furnished  the  street  as 
a  whole.  We  do  not  judge  the  lighting  of  a  street  by 
the  darkest  spots  so  much  as  by  the  general  effect.  A 
large  number  of  small  lights  give  an  impression  of  din- 
giness  unless  the  aggregate  candle-power  be  large.  A 
few  large  lights,  on  the  other  hand,  give  in  places  a  bril- 
liant effect,  although  the  minimum  illumination  may  be 
rather  small. 

If  artificial  illuminants  gave  a  spherical  distribution 
of  light  the  computation  of  street  lighting  would  be 
easy,  but  as  has  already  been  seen,  they  do  not.  What 


248  THE   ART    OF   ILLUMINATION. 

is  worse,  the  nominal  brilliancy  is  generally  not  found  in 
practice. 

For  years  open  arc  lamps-  have  been  classified  as  2000- 
cp  or  "  full  arc,"  and  as  i2OO-cp,  or  "  half-arc  "  lamps, 
but  these  alleged  candle-powers  are  never  obtained  even 
in  the  direction  of  maximum  illumination.  The  former 
are  lamps  taking  about  9.5  to  10  amperes,  and  450  to 
480  watts,  the  latter  6.5  to  7  amperes  and  325  to  350 
watts.  Their  actual  maximum  intensities  are,  respect- 
ively, about  1 200  cp.  and  700  cp.,  located  at  about  45 
degrees  below  the  horizontal  plane.  Reduced  to  mean 
spherical  measures  their  ordinary  intensities  are  about 
600  and  300  cp.,  respectively.  In  the  horizontal  plane 
these  intensities  fall  to  about  350  cp  and  200  cp. 

The  result  of  this  distribution  is  a  zone  of  very  bril- 
liant light  in  a  circle  having  its  radius  equal  to  about  the 
height  of  the  light,  comparatively  weak  light  within  it, 
and  rather  feeble  light  at  greater  distances  where  the  rays 
are  more  nearly  horizontal.  Fig.  96  shows  this  illumi- 
nated ring  with  startling  distinctness.  The  dark  area 
within  is  exaggerated  by  the  effect  of  a  dirty  globe,  the 
globes  on  open  arc  lamps  being  accessible  to  dust  and 
hence  difficult  to  keep  clean. 

More  conspicuous  even  than  the  dark  space  is  the  nar- 
rowness of  the  area  of  brilliant  illumination,  and  the 
rapid  fading  away  into  darkness  of  such  details  as  fhe 
picket  fence  merely  across  the  sidewalk  from  the  pole  by 
which  is  carried  the  mast-arm  for  the  arc  lamp.  This 
was  a  so-called  2OOO-cp  arc,  not  kept  in  the  best  order, 
to  be  sure,  but  still  no  worse  than  can  easily  be  found  in 
any  town  where  there  are  many  open  arc  lamps. 

Obviously  one  cannot  compute  the  illumination  from 
such  a  lamp  on  the  theory  that  the  distribution  is  even 


EXTERIOR    ILLUMINATION.  249 

approximately  spherical.  In  order  to  calculate  the 
effect  it  is  necessary,  instead  of  assuming  L  in  the 
formulas  to  be  constant,  to  treat  it  as  a  variable,  and  to 
use  its  real  value  at  each  angle  below  the  horizontal  in 


Fig.  96. —  Shadow  below  Open  Arc. 

computing  the  illuminating  effect  at  various  points.  In 
other  words,  the  illumination  at  distant  points  must  be 
made  with  reference  to  the  luminous  distribution  curve 
of  the  lamps. 

Fig.  97  gives  a  set  of  distribution  curves  for  the  forms 
of  arc  lamps  in  most  frequent  use. 

Curve  A  is  from  a  series  alternating-current  enclosed- 
arc  lamp  taking  425  watts  with  a  current  value  of  6.6 
amperes.  It  was  fitted  with  the  usual  opalescent  inner 


250 


THE   ART    OF   ILLUMINATION. 


globe  and  a  clear  outer  globe,  and  was  used  with  a  solid 
upper  and  cored  lower  carbon. 

Curve  B  is  from  the  ordinary  open  continuous-cur- 
rent arc,  taking  330  watts  at  6.6  amperes,  and  including 


Fig.  97.— Distribution  of  Light  from  Arcs. 

the  usual  clear  glass  globe.     This  is  the  arc  nominally 
rated  at  1200  cp. 

Curve  C  is  from  the  9.6-ampere  48o-watt  arc  usually 


EXTERIOR    ILLUMINATION.  251 

classed  as  2000  cp.  Like  the  preceding,  it  is  a  con- 
tinuous current  arc  with  clear  glass  globe. 

Curve  D  is  from  a  continuous-current  series  arc 
lamp  taking  6.6  amperes  and  480  watts.  It  was  pro- 
vided with  an  opalescent  inner  and  a  clear  outer  globe, 
and  was  worked  with  both  carbons  solid. 

Lamps  A  and  D  were  provided  with  reflecting  shades 
to  turn  outwards  and  downwards  the  light  ordinarily 
thrown  upwards.  Lamps  B  and  C  could  not  be  thus 
treated  for  the  reason  that  so  little  light  is  thrown 
above  the  horizontal  that  a  reflector  is  practically 
useless. 

All  these  curves  are  from  tests  made  on  commercial 
arc  lamps  of  recent  manufacture  at  the  same  period  and 
by  the  same  experimenters  in  the  laboratory  of  one  of 
the  large  electrical  companies,  which  manufactures 
lamps  of  each  of  these  types.  As  the  tests  were  at  the 
time  made  not  for  publication,  but  for  the  private  use 
of  the  company's  engineers,  and  under  uniform  condi- 
tions, the  author  believes  them  to  be  more  reliable,  par- 
ticularly as  regards  the  comparative  results  from  the 
several  lamps,  than  even  the  average  of  tests  made  by 
different  observers  on  lamps  under  diverse  conditions. 
Such  curves  are  of  necessity  only  approximate,  because 
of  the  variations  due  to  differing  adjustments  and  to  the 
peculiarities  of  different  lamps. 

NOWT  looking  at  curve  C,  for  example,  we  may  repeat 
the  computation  made  with  respect  to  Fig.  95,  using  the 
real  value  of  the  candle-power  instead  of  the  assumed 
value. 

Taking  the  arcs  as  25  ft.  above  the  ground,  and  com- 
puting the  illumination  at  100  ft.  from  the  pole,  the  re- 
quired ray  is  depressed  14  degrees  below  the  horizontal 


252  THE   ART   OF   ILLUMINATION. 

plane.  The  corresponding  candle-power  from  curve  C 
is  about  620,  so  that  putting  this  value  for  L 

/  =         '       =  0.058  candle-foot, 

instead  of  0.094  candle-foot  as  in  the  previous  example. 
If  we  take  d  =  150  ft.,  the  result  is  still  more  unsatis- 
factory, for  /  =  0.0433  with  the  assumed  1000  cp,  but 
since  in  this  case  a  =  9  degs.  30  min.,  L  is  really  only 
490,  whence  /  =  0.0168  candle-foot. 

It  therefore  becomes  evident  that  with  the  distribu- 
tion curve  of  C  it  is  very  easy  to  get  good  illumination 
not  far  from  the  lamp,  but  that  at  distances  from  the 
lamp  of  four  or  five  times  the  height  of  the  lamp  the 
illumination  is  very  deficient.  A  glance  at  curve  B 
shows  a  distribution  curve  of  very  similar  shape,  and,  in 
fact,  all  open  arcs  have  the  common  weakness  of  giving 
more  light  than  is  necessary  near  the  pole  and  too  little 
far  away  from  it.  The  distribution  would  be  much  im- 
proved if  the  curve  could  be  swung  upward  about  20 
degs.  toward  the  horizontal. 

If  there  were  any  convenient  way  of  getting  a  distribu- 
tion that  would  give  uniform  illumination  up  to  a  rea- 
sonable distance  it  would  be  extremely  useful.  The 
polar  equation  for  such  a  distribution  curve  would  be 

ut 

J-i    —          r~s 


Fig.  98  shows  this  curve  plotted  for  h  =  25  and 
/  =  o.i  candle-foot.  The  theoretical  curve  obviously 
becomes  asymptotic,  which  no  practical  curve  could  ever 
do  under  finite  conditions,  but  it  is  not  outside  the 
bounds  of  possibility  to  construct  a  reflecting  and  re- 
fracting system  after  the  pattern  of  the  holophane  globe, 


EXTERIOR    ILLUMINATION. 


253 


which  should  send  out  at  angles  not  far  from  the  hori- 
zontal most  of  the  light  which  is  now  wasted  in  need- 
lessly brilliant  illumination  near  the  light. 

Unhappily  the  arc  slowly  changes  place  in  common 
lamps  as  the  carbons  are  consumed,  so  as  to  interfere 


Fig.  98. — Ideal  Distribution  Curve. 

with  the  use  of  such  a  device.  But  it  would  be  quite 
feasible  with  a  focusing  lamp,  or  with  Nernst  or  similar 
lamps,  although  globes  of  the  kind  in  question  are  not 
easy  to  keep  clean,  and  would  have  to  be  kept  dust-tight 
to  obtain  the  best  results. 

Now,  when  we  remember  that  the  curves  of  Fig.  97 
represent   the   lamps    in   their   respective   best   working 


254  THE   ART   OF   ILLUMINATION. 

conditions,  and  not  when  the  globes  are  ill-cared  for  or 
the  arc  abnormal  in  length  and  position,  it  becomes  evi- 
dent that  if  any  attention  is  to  be  paid  to  minimum 
illumination,  even  the  most  powerful  commercial  arcs 
cannot  be  very  widely  spaced.  At  400  ft.  spacing  the 
midway  point  receives  from  an  ordinary  i2OO-cp  arc  25 
ft.  high  just  about  o.oi  candle-foot,  which,  for  most 
practical  purposes,  is  no  light  at  all.  Only  by  raising 
the  lamp  high  enough  to  take  advantage  of  the  light  at 
angles  further  from  the  horizontal  can  adequate  light 
at  such  a  distance  be  obtained,  and  this  is  an  impractica- 
ble expedient  on  account  of  the  cost  and  trouble  and  the 
interference  of  trees  and  other  obstacles.  For  with  such 
arcs  the  maximum  illumination  is  obtained  when  the 
height  of  the  lamp  is  about  seven-tenths  the  distance  to  the 
point  to  be  lighted.  The  tower  system  of  lighting,  used 
extensively  in  this  country  fifteen  years  ago,  but  now  prac- 
tically abandoned,  was  the  result  of  this  consideration. 

The  important  thing  to  be  decided  as  a  basis  for  all 
computations  on  street  illumination  is  the  amount  of 
light  required.  There  is  little  general  agreement  on 
this  important  matter.  We  may  get  an  idea  of  the  mag- 
nitudes to  be  employed  by  remembering  that  moonlight 
in  the  latitude  of  the  Northern  States  is  generally  from 
o.oi  to  0.03  candle-foot.  This  intensity  is  of  considerable 
service  at  the  maximum  limit,  but  of  little  use  at  the 
minimum.  When  anything  like  adequate  illumination 
is  to  be  furnished  the  minimum  should  not  be  less  than 
0.03  candle-foot.  With  this  minimum  derived  from  arc 
lamps  the  street,  as  a  whole,  will  be  brilliantly  lighted. 
Under  ordinary  circumstances  only  principal  streets 
would  be  lighted  to  this  extent,  and  elsewhere  the  mini- 
mum mig-ht  fall  somewhat  lower. 


EXTERIOR    ILLUMINATION.  255 

To  determine  the  real  illumination  produced  at  any 
distance  by  a  particular  radiant,  it  is  necessary  to  take 
the  assumed  height  and  the  distribution  curve  for  the 
radiant,  and  then  to  compute  the  illumination  at  that 
distance,  using  the  real  candle-power  corresponding  to 
the  direction  of  the  ray  considered.  To  save  labor  it  is 
convenient  to  plot  the  illumination  at  various  distances 
in  the  form  of  a  curve,  thus  enabling  the  illumination  in 
candle-feet  to  be  read  directly. 

Fig.  99  shows  a  set  of  curves  thus  computed  from  the 
four  types  of  arc  lamps,  of  which  distribution  curves 
were  shown  in  Fig.  97.  The  first  important  lesson  to 
be  drawn  from  them  is  that  none  of  the  lamps  shown 
gives  really  useful  illumination  at  a  distance  of  more 
than  150  ft.,  and  that  lamps  A  and  B  should  not  be 
spaced  more  than  225  ft.  apart  if  the  minimum  illumina- 
tion is  to  be  about  0.03  candle-foot.  Lamps  C  and  D 
might  be  spaced  at  300  ft.,  but  not  much  further  with- 
out producing  a  conspicuous  dark  belt. 

A  and  D,  the  enclosed  arcs,  give  relatively  better  illu- 
mination at  considerable  distances  than  the  open  arcs. 
It  should  be  noted  that  the  height  of  all  lamps  is  as- 
sumed to  be  25  ft.,  and  that  the  curves  begin  at  50  ft. 
from  the  lamp,  the  space  within  that  distance  being 
relatively  well  lighted. 

Comparing  the  several  types  of  lamps,  it  at  once  ap- 
pears that  for  street  lighting  the  6.6-ampere  enclosed 
continuous  current  arc  is  fully  equal  to  the  9.6-ampere 
open  arc  (so-called  2OOO-cp),  while  the  6.6-ampere  alter- 
nating arc  is  rather  better  than  the  6.6  ampere  con- 
tinuous-current open  arc  (so-called  i2OO-cp),  but  is 
materially  less  effective  than  the  other  two.  In  the  au- 
thor's opinion,  it  would  correspond  to  a  nominal  1500 


CANDLE  FEET        I* 


EXTERIOR    ILLUMINATION.  257 

cp  as  respects  the  open  arcs  rated  in  the  old-fashioned 
way. 

But  theory  and  experience  unite  in  indicating  that 
effective  illumination  on  streets  is  not  measured  exactly 
by  the  process  just  explained,  useful  though  it  may  be. 
There  is  a  physiological  as  well  as  a  physical  factor  in 
illumination,  for  in  the  presence  of  lights  of  great  in- 
trinsic brilliancy  the  iris  closes  and  the  image  on  the 
retina  grows  faint,  like  the  image  in  a  camera  when  the 
lens  is  stopped  down. 

Thus  it  is  difficult  to  see  beyond  a  brilliant  light,  and 
when  the  eye  is  exposed  to  the  intense  glare  of  an  open 
arc  it  does  not  recover  promptly  enough  in  passing  on 
to  get  the  full  value  of  the  relatively  feeble  light  at  a  dis- 
tance from  the  lamp.  This  effect  and  the  more  uniform 
distribution  of  light  account  for  the  well-established  fact 
that  enclosed  arcs,  for  both  alternating  and  continuous 
currents,  give  more  satisfactory  lighting  than  would 
seem  to  be  warranted  by  the  computed  intensity  of  the 
illumination.  Besides,  the  enclosed  arcs  are  generally 
steadier,  which  also  tends  greatly  to  improve  matters. 

The  same  advantages  would  be  secured  by  using  open 
arcs  with  properly  designed  diffusing  globes,  but  these 
last  really  require  the  employment  of  a  focusing  lamp  in 
which  both  carbons  are  fed  into  the  arc  at  rates  propor- 
tional to  their  rates  of  consumption,  so  as  to  hold  the 
arc  in  a  fixed  position.  Such  lamps  are  more  compli- 
cated and  less  fitted  to  outdoor  use  than  ordinary  lamps, 
and  have  not  come  into  use  for  this  purpose  in  this 
country. 

The  advantages  in  uniformity  of  distribution  and  in 
absence  of  shadowed  areas,  gained  by  the  use  of  en- 
closed arcs,  are  very  conspicuous.  Fig.  100  shows  the 


258  THE   ART   OF   ILLUMINATION. 

illumination  from  an  ordinary  open  arc  in  excellent 
operative  condition,  aided  by  a  globe  ground  below  to 
diffuse  the  light  and  to  lessen  the  shadows  beneath  the 
arc.  In  spite  of  this  the  illumination  is  far  from  uniform, 
and  the  side  rods  of  the  lamp,  on  account  of  the  small 
luminous  area  of  the  arc,  throw  dense  lateral  shadows. 
Fig.  101  shows  the  result  of  replacing  this  particular 
lamp  by  an  enclosed  arc.  The  shadows  completely  dis- 


Fig.  ioo. — Open  Arc  at  Its  Best. 

appear  and  the  more  powerful  rays  near  the  horizontal 
are  shown  both  by  the  better  illumination  along  the 
street  and  by  the  glare  that  evidently  entered  the 
camera.  These  results  are  very  striking,  and  amply 
justify  the  present  tendency  to  replace  open- by  enclosed 
arcs,  quite  irrespective  of  the  lessened  cost  of  carbons 
and  of  trimming  in  the  latter  case. 


EXTERIOR    ILLUMINATION. 


259 


Enclosed  continuous-current  arcs  can  be  operated  for 
about  100  hours  with  one  pair  of  carbons;  in  other 
words,  they  have  to  be  trimmed  only  about  once  a  week 
in  all-night  street  lighting,  while  the  ordinary  open  arcs 
require  trimming  daily  under  like  circumstances.  The 
resulting  saving  in  labor  and  in  carbons  is  variously  esti- 
mated, and  changes  somewhat  with  local  conditions,  but 
the  weight  of  the  evidence  indicates  a  net  saving  of  about 
$10  per  year  per  lamp,  the  same  energy  being  used  in 
each  case. 

Of  late  a  strong  tendency  has  developed  toward  the 


Fig.  101. — Light  from  Enclosed  Arc. 

use  of  alternating-current  lamps,  worked  in  series  like 
the  others  through  the  aid  of  constant-current  trans- 
formers or  automatic  regulators,  which  take  the  alter- 
nating current  at  constant  voltage  and  deliver  a  current 
constant  in  amount,  but  varying  in  voltage  according  to 
the  load  to  be  carried.  To  discuss  these  interesting 


260  THE   ART   OF   ILLUMINATION. 

mechanisms  in  detail  is  without  the  purpose  of  this 
volume.  Suffice  it  to  say,  that  they  do  their  work  ex- 
tremely well,  although  at  the  cost  of  certain  inconven- 
iences, less  on  the  whole  than  those  attending  the  use  of 
ordinary  arc  generators. 

As  has  already  been  noted,  the  alternating  arcs  de- 
mand a  little  more  energy  relatively  than  continuous- 
current  arcs,  but  as  a  rule  this  loss  is  fully  compensated 
by  the  more  economical  distribution  of  current  rendered 
possible.  The  enclosed  alternating  arcs  require  slightly 
more  expensive  carbons,  and  rather  more  frequent  trim- 
ming than  the  enclosed  continuous-current  arcs.  As  a 
rule  one  pair  of  carbons  will  last  seventy-five  or  eighty 
hours,  and  the  street  lamps  must  be  trimmed  once  in 
five  or  six  days,  but  a  considerable  saving  in  carbons  and 
labor  is  still  effected.  It  probably  amounts  in  average 
cases  to  about  $8  per  year  per  lamp. 

The  smaller  intrinsic  brilliancy  of  enclosed  arcs  en- 
ables some  gain  to  be  made  by  placing  them  lower  than 
25  ft.,  perhaps  no  more  than  1 8  or  20  ft.,  which  means 
a  still  further  gain  in  effective  lighting.  As  between 
continuous  current  and  alternating-current  arcs  taking 
the  same  current,  the  former  are  distinctly  more  power- 
ful; but  either  at  6.6  amperes  can  replace  the  open  arcs 
either  of  1200  or  2000  nominal  candle-power,  lamp  for 
lamp,  and  give  about  equally  satisfactory  illumination. 
In  new  installations  the  alternating  lamps  may  advan- 
tageously be  spaced  a  little  closer  than  the  continuous- 
current  enclosed  arcs,  as  the  curves  of  Fig.  99  in- 
dicate. 

There  is  no  question  that  where  plenty  of  light  is 
wanted  in  a  street  comparatively  clear  of  trees,  well  dis- 
tributed arc  lights  give  by  far  the  best  results  yet  at- 


EXTERIOR    ILLUMINATION.  261 

tained.  But  where,  as  in  many  residence  streets  in  the 
smaller  cities,  the  whole  roadway  is  well  shaded  by  trees, 
arc  lamps  as  ordinarily  installed  find  their  usefulness 
greatly  limited  by  shadows.  If  the  foliage  does  not 
come  too  low  good  results  can  be  obtained  by  putting 
the  arcs  on  cross  suspensions  over  the  center  of  the 
street  at  a  height  of  not  over  1 8  to  20  ft.  In  this  case 
the  enclosed  or  otherwise  shaded  arcs  have  an  immense 
advantage,  as  they  can  thus  be  swung  low  without  seri- 
ously dazzling  the  eyes,  and  throw  shadows  far  less  in- 
tense than  the  open  arcs. 

But  there  are  streets  so  well  shaded  that  even  arcs  so 
placed  are  at  a  disadvantage,  and  there  are  also  many 
cases  in  which  there  is  no  real  need  of  a  brilliant  illumi- 
nation, but  merely  enough  light  is  desired  to  make  the 
way  fairly  clear.  Economy  also  sometimes  dictates 
caution  in  the  expenditures  for  street  lighting,  and  in 
such  cases  incandescent  electric  lamps  or  gas  lamps  are 
capable  of  doing  good  service  at  comparatively  moder- 
ate cost.  The  incandescent  lamps  used  for  such  service 
are  nearly  always  operated  in  series,  either  on  the  same 
circuits  as  arc  lamps  or  on  separate  series  circuits  .by 
themselves. 

In  the  former  case  the  lamps  are  generally  50,  65,  or 
100  candle-power,  made  to  take  a  constant  current  cor- 
responding to  that  used  for  arcs,  and  it  should  be  noted 
that  such  lamps  are  costly  in  the  matter  of  renewals  and 
difficult  to  operate  satisfactorily.  In  the  latter  case  the 
incandescents  are  generally  of  16,  25,  or  32  candle- 
power,  worked  in  series  upon  an  alternating-current  cir- 
cuit of  1000  or  2000  volts,  taking  2  to  4  amperes.  The 
running  conditions  in  either  case  are  rather  severe,  ow- 
ing to  the  likelihood  of  fluctuations  in  current,  and  in- 


262 


THE   ART   OF   ILLUMINATION. 


candescent  lamps  intended  for  such  service  are  rarely  of 
higher  initial  efficiency  than  3.5  to  4  watts  per  candle. 

In  the  case  of  gas  lighting,  the  present  tendency  is  to 
use  mantle  burners  giving  initially  50  to  100  candle- 
power.  Their  color  is  comparatively  inoffensive  in 
street  lighting,  and  they  can  be  run  at  very  low  cost,  but 
outdoor  conditions  seem  to  tend  to  very  rapid  deteriora- 
tion of  the  mantles,  so  that  in  practice  it  is  difficult  to 
find  a  street  in  which  most  of  the  mantles  have  not  long 


0.2 


0.1 


1PJ 


20'  30'  40'  50'  60' 

Fig.  102. — Illumination  from  Incandescent  Lamps. 

outlived  their  usefulness.  Incandescent  electric  lamps 
deteriorate  fast  enough,  but  even  in  street  service  they 
hold  their  brilliancy  much  better  than  the  mantle  burn- 
ers. The  moral  is  that  in  using  either  of  these  illumi- 
nants,  a  very  liberal  allowance  should  be  made  for  falling 
off  in  candle-power. 

As  in  case  of  arcs,  the  illumination  should  be  com- 
puted from  the  distance,  height,  and  light-distribution 
curve  of  the  radiant.  For  incandescent  lamps  of  25,  50, 
and  100  candle-power  the  resulting  illumination  curves 
are  shown  in  Fig.  102,  which  may  be  compared  with 


EXTERIOR    ILLUMINATION.  263 

Fig.  99.  It  is  clearly  evident  therefrom  that  such  lights 
should  hardly  be  spaced  over  120  ft.  apart,  even  when  of 
100  candle-power,  while  those  of  50  and  25  candle- 
power  should  be  spaced  much  nearer.  If  they  held 
their  brilliancy  well,  one  might  space  them  further,  but, 
in  fact,  it  is  undesirable. 

In  using  the  above  curves  for  mantle  burners  it 
should  be  noted  that  one  such  burner  corresponds  ap- 
proximately to  a  5O-cp  incandescent  electric  lamp,  being 
somewhat  better  in  the  early  stages  of  its  life,  but  losing 
brilliancy  rather  more  rapidly. 

In  heavily  shaded  streets  incandescents  and  mantle 
burners  may  best  be  bracketed  out  a  few  feet  from  the 
curb,  with  alternate  lamps  on  opposite  sides  of  the 
street;  loo-cp  lamps  spaced  in  this  way,  with  200  ft.  be- 
tween consecutive  lamps  on  the  same  side,  or  even  250 
ft.  under  favorable  conditions,  will  produce  a  tolerably 
lighted  street,  and  5O-cp  lamps  spaced  a  little  closer,  say, 
at  150  to  200  ft.  on  each  side,  will  do  even  better,  but 
except  for  use  in  shaded  streets  arcs  are  generally  to  be 
preferred. 

When  close  economy  is  an  object,  25-cp  lamps  spaced 
at  150  ft.  between  lamps  on  the  same  curb,  or  mantle- 
burners  similarly  placed,  will  give  tolerable  lighting,  but 
when  in  suburban  districts  lights  must  be  economized,  it 
is  better  to  trust  to  arcs  spaced  even  so  widely  as  to 
leave  regions  of  comparative  darkness  between  them. 

Whenever  possible,  lights  should  be  so  located  at 
street  corners  as  to  shine  effectively  in  all  four  direc- 
tions. They  should  be  placed  preferably  on  long  mast- 
arms  or  on  cross-suspensions,  unless  in  a  fairly  clear 
street,  where  pole  tops  or  short  brackets  are  effective. 
One  small  but  useful  point  in  locating  lights  is  as.fol- 


264  THE   ART    OF   ILLUMINATION. 

lows:  Never  locate  a  lamp,  particularly  an  arc,  at  the 
curb  squarely  opposite  a  crossing,  for  the  shadow  of  one 
walking  on  the  crossing  plunges  his  way  into  inky  black- 
ness as  soon  as  he  has  passed  the  light. 

Squares  and  open  spaces  should  be  treated  somewhat 
in  the  same  manner  as  streets,  but  if  the  spaces  are  clear 
the  lights  may  well  be  placed  higher  than  in  street  light- 
ing, 30  to  40  ft.  being  advantageous.  In  well  shaded 
spaces  one  may  use  lights  of  small  intensity  to  con- 
siderable advantage,  but  ordinarily  arcs  do  the  best 
work. 

As  to  intensity,  the  lighting  depends  on  the  character 
of  the  space,  but  it  should  never  be  less  than  in  a  well- 
lighted  street.  Now  and  then  such  lighting  drifts  natu- 
rally into  a  species  of  scenic  illumination,  the  object 
being  to  bring  some  fine  square  into  brilliant  relief.  Oc- 
casionally in  such  work  incandescent  lamps  can  be 
massed  with  admirable  results,  although  in  general  it  is 
not  advisable  to  mix  arc  and  incandescent  lighting. 

The  arrangement  of  public  lights  should  be  more  or 
less  influenced  by  the  general  illumination  of  the  private 
premises  along  the  streets.  In  large  cities  it  often 
happens  that  through  the  evening  hours,  when  the  citi- 
zens are  much  abroad,  the  sidewalks  are  fully  lighted  by 
the  stray  illumination  from  windows.  In  such  instances 
the  public  lighting  can  be  well  directed  toward  special 
points  where  it  will  do  the  most  good,  care  being  always 
taken  to  see  that  the  illumination  does  not  fall  below 
minimum  requirements  when  the  mass  of  private  lights 
is  out.  In  small  cities  there  is  comparatively  little  aid 
of  this  sort.  The  most  troublesome  problems  are  those 
connected  with  the  environs  of  such  cities  where  miles  of 
sparsely  settled  streets  must  be  dealt  with. 


EXTERIOR    ILLUMINATION.  265 

The  practical  problem  of  adequately  lighting  the 
streets  of  a  city  is  one  which  requires  the  local  data  for 
its  solution.  The  amount  and  distribution  of  streets 
and  the  needs  and  distribution  of  the  population  are  the 
controlling  factors  in  the  matter,  and  obviously  these 
vary  greatly  from  place  to  place. 

Then,  too,  the  cost  of  lights  and  the  funds  available 
for  the  purpose  depend  on  local  conditions.  In  par- 
ticular, the  price  of  lights  varies  so  much  that  it  is  diffi- 
cult even  to  strike  an  average.  Few  topics  offer  less 
chance  for  certitude  or  are  more  unsatisfactory  to  in- 
vestigate. It  makes  a  great  difference  whether  full  arcs 
or  half  arcs  (so-called)  are  in  use,  or  incandescents,  or 
Welsbachs;  also  whether  the  lights  are  burned  all  night 
and  every  night,  every  night  until  midnight,  or  on 
"moonlight  schedule,"  that  is,  on  all  nights  and  at  all 
hours  of  the  night  when  there  is  not  clear  moonlight. 

In  large  cities  the  tendency  is  toward  powerful  arc 
lights  burning  every  night  and  all  night,  supplemented 
by  incandescent  lamps  and  by  gas  burners.  In  smaller 
cities  and  towns  the  smaller  arcs  are  apt  to  be  used, 
burned  either  on  moonlight  schedule  or  until  12  or  i 
o'clock. 

In  the  latitude  of  the  Northern  United  States  "  all 
night  and  every  night "  public  lighting  usually  means 
from  3900  to  4200  hours  of  lighting  yearly,  according 
to  the  treatment  of  twilight  and  dawn,  and  the  weather. 
The  moonlight  schedule  is  carried  out  with  various 
slight  modifications,  and  evidently  depends  considerably 
upon  the  weather,  but  in  practice  it  amounts  to  not  far 
from  3000  hours  per  year,  while  lights  run  from  dusk  to 
midnight,  or  I  a.  m.,  usually  will  burn  2000  to  2200 
hours  per  year.  All  night  and  every  night  lighting  is, 


266  THE   ART    OF   ILLUMINATION. 

of  course,  the  thing  to  be  desired,  but  if  economy  is 
necessary,  places  of  moderate  size  can  be  very  satis- 
factorily lighted  on  a  liberally  administered  moonlight 
schedule. 

As  between  "  full  "  and  "  half "  arcs,  the  advantage 
economically  lies,  on  the  whole,  with  the  former,  pro- 
vided the  same  total  illumination  is  to  be  obtained  in 
each  case.  The  following  table  gives  the  spacing  re- 
quired for  various  radiants  on  an  assumed  minimum 
illumination  of  0.02  candle-foot  at  a  point  midway  be- 
tween lights: 

DISTANCE  LIGHTS 

KIND  OF  LIGHT.  BETWEEN  LIGHTS.     PER  MILE. 

6. 6-ampere  enclosed  D.  C.  arc 340  15 

9. 6-ampere  open  D.  C.  arc 315  17 

6. 6-ampere  enclosed  A.  C.  arc. .      275  19 

6.6-ampere  open  D.  C.  arc 260  20 

5O-cp    incandescent  lamp,    electric    (or 

mantle  burner) 100  53 

The  relative  expense  of  lighting  by  these  various 
radiants  depends  in  great  measure  upon  circumstances  not 
to  be  predicted.  The  price  charged  by  lighting  companies 
for  public  lighting  is  simply  whatever  the  community 
will  stand. 

The  relations  between  lighting  companies  and  munici- 
palities are,  in  most  cases,  mutually  predatory.  The 
former,  having  acquired  and  utilized  a  public  franchise 
can,  in  a  measure,  hold  the  street  against  competition, 
and  can  maintain  prices  at  the  highest  figure  that  can  be 
juggled  through  the  city  government  without  public 
scandal.  The  latter,  through  the  earnest  efforts  of  its 
practical  politicians,  can  worry  and  threaten  the  former 
into  yielding  up  a  substantial  annual  rake-off  in  the  form 
of  jobs  for  heelers,  contributions  to  campaign  funds,  or 
plain  cash.  Occasionally  simple  and  decent  business 


EXTERIOR    ILLUMINATION.  267 

relations  are  maintained,  but  seldom  for  any  great 
length  of  time. 

The  prices  actually  charged  for  public  arc  lights 
burned  all  night  and  every  night  range  very  widely,  but 
usually  between  $75  and  $125  per  lamp  per  year.  The 
former  price  is  seldom  reached  or  discounted,  save  in 
stations  operated  by  water  power  or  under  very  strong 
competition,  and  even  then  generally  is  only  for  "  half " 
arcs.  The  latter  price  is  seldom  exceeded,  save  where 
underground  distribution  is  demanded  or  in  case  of  very 
scattered  service. 

Incandescent  lamps  of  50  candle-power,  or  there- 
abouts, usually  bring  $30  to  $35  per  year,  and  the 
former  figure  is  a  common  one  for  mantle  burners  on  the 
gas  system.  For  equal  minimum  intensity  of  illumina- 
tion there  is  not  much  to  choose  between  the  several 
illuminants  at  the  prices  mentioned,  the  choice  between 
them  being  due  to  suitability. 

At  the  same  total  cost,  however,  the  arc  lights  give  a 
considerably  higher  average  illumination,  and  experi- 
ence shows  that  on  the  whole  the  arcs,  which  have  to  be 
inspected  at  frequent  intervals  for  the  purpose  of  trim- 
ming, are  kept  nearer  their  point  of  maximum  efficiency 
than  either  incandescents  or  gas  burners. 

The  real  cost  of  public  lighting  is,  of  course,  im- 
mensely variable,  since  amount  and  price  are  both  varia- 
ble. In  the  average  New  England  city  most  of  the 
lighting  is  by  arcs,  and  there  is  an  average  of  one  arc  for 
each  175  to  200  inhabitants.  The  total  cost  of  public 
lighting  is  frequently  from  50  cents  to  $i  per  inhabitant, 
and  sometimes  rises  to  $1.50,  and  even  to  $2  per  in- 
habitant. It  is,  therefore,  no  inconsiderable  item  of 
public  expense. 


268  THE   ART    OF   ILLUMINATION. 

Whether  street  lighting  should  be  done  by  contract 
with  a  corporation  or  directly  by  the  municipality  is  one 
of  the  mooted  questions  of  economics.  In  theory,  it 
would  seem  that  such  a  public  service  should  be  done  by 
the  city  or  town  itself,  on  the  same  principle  that  all 
towns  own  their  sewerage  systems  and  most  own  their 
waterworks.  But  while  experience  seems  to  have 
shown  that  public  waterworks  are  in  every  way  desira- 
ble, the  same  cannot  be  said  of  gas  and  electric  plants. 
The  arguments  pro  and  con  are  about  the  same  in  each 
case,  yet  the  average  results  seem  to  be  different.  In 
some  municipal  lighting  plants,  mostly  small,  the  eco- 
nomic results  have  been  excellent,  in  most  they  have 
been  unmistakably  bad. 

If  a  municipality  could  start  in  de  iwvo  and  erect  its 
complete  lighting  system  according  to  the  best  modern 
practice,  and  run  that  system  anything  like  as  economic- 
ally as  it  would  be  run  by  a  private  company,  it  could 
unquestionably  do  its  public  lighting  at  a  great  saving 
in  the  majority  of  cases. 

But  it  can  rarely  start  and  operate  thus  freely.  It 
often  is  compelled  by  law  to  buy  out  the  existing  plant, 
generally  at  a  price  far  larger  than  would  suffice  to  erect 
a  modern  plant,  for  the  art  has  been  changing  rapidly. 
It  is  lucky  if  a  plant,  old  or  new,  can  be  secured  without 
furnishing  pickings  and  stealings  for  somebody,  and  in 
its  operation  the  finger  of  the  politician  is  too  often  in- 
serted for  no  honest  purpose. 

In  this  matter  of  municipal  plants  statistics  are  even 
more  utterly  mendacious  than  usual,  and  conceal  rather 
than  disclose  the  facts  in  the  case.  With  respect  to 
electrical  plants  at  least,  however  owned,  there  is  nearly 
always  deliberate  concealment  or  entire  neglect  of  de- 


EXTERIOR    ILLUMINATION.  269 

preciation,  both  that  due  to  wear  and  tear  and  the  larger 
amount  due  to  improvements  in  the  art,  so  that  from  the 
books  or  reports  it  is  almost  impossible  to  figure  the 
actual  cost  of  furnishing  the  electrical  energy. 

The  matter  may,  on  the  whole,  be  summed  up  about 
as  follows:  If  a  municipality  could  both  acquire  a  light- 
ing plant  and  operate  it  at  the  ordinary  current  rates  for 
apparatus,  labor,  and  material  paid  by  private  buyers 
and  employers,  it  could  effect  a  large  saving  in  its  cost 
of  public  lighting;  but  if  the  plant  is  touched  by  venal 
politics  it  will  assuredly  prove  a  costly  failure. 

Many  improvements  are  possible  in  street  lighting, 
but  for  the  present  the  arc  lamp  must  be  the  main  re- 
liance. At  first  thought  the  Nernst  lamp  would  seem 
to  offer  advantages,  but  it  does  not  readily  lend  itself  to 
the  distribution  in  series  which  is  desirable  in  street 
lighting  for  the  sake  of  economy.  Improvements  in 
mantle  gas  burners  may  bring  them  into  a  position  of 
great  usefulness,  which  they  have  not  yet  attained  by 
reason  of  their  rapid  deterioration.  But  for  the  most 
part  the  electric  arc  is  the  best  available  source  of  light. 

Contracts  for  arc  lighting  should  never  be  drawn  on 
the  basis  of  a  nominal  candle-power.  They  should 
clearly  specify  the  kind  of  arc  to  be  installed^  the  amount 
of  energy  to  be  taken  in  each  arc,  and  the  kind  of  shades 
to  be  used.  The  nature  of  the  fixtures  should  be  spe- 
cifically designated,  whether  pole  tops,  brackets,  mast- 
arms,  or  cross  suspensions.  These  and  the  locations  of 
the  lamps  should  be  designated  by  someone  familiar 
with  practical  street  lighting,  following  the  general  line 
of  the  data  which  have  here  been  given.  The  hours  of 
lighting  should  be  distinctly  stated,  with  rebates  for 
failure  to  provide  continuous  light  within  these  hours. 


27o  THE   ART    OF   ILLUMINATION. 

Such  rebates  should  be  merely  nominal  for  deficiencies 
up  to,  say,  i  or  2  per  cent,  of  the  total  hours  of  lighting, 
and  punitive  on  an  increasing  scale  for  greater  de- 
ficiencies. 

The  fixtures  used  for  street  lighting  are  of  very  vari- 
ous patterns,  but  fall  into  four  general  classes :  pole-tops, 
brackets,  mast-arms,  and  cross-suspensions.  These  have 
been,  with  the  exception  of  the  mast-arm,  in  use  for 
public  lighting  for  a  very  long  period,  going  back  to  the 
days  of  oil  lamps  and  candles.  The  pole-top  fixture  is 
essentially  a  support  for  a  lamp  on  the  top  of  a  post.  In 
arc  lighting  it  is  generally  combined  with  a  protecting 


Fig.  103. — Pole-top.  Fig.  104. — Bracket. 

weather  hood  to  shield  the  top  of  the  lamp  from  the 
weather,  and  also  sometimes  to  shield  the  individual 
switch  for  short-circuiting  the  lamp.  Fig.  103  shows  a 
typical  pole-top  such  as  is  often  used  for  enclosed  arc 
lamps.  The  thin  side  rods  are  placed  edge  toward  the 
arc  to  obviate  shadows,  and  the  whole  affair  fits  neatly 


EXTERIOR    ILLUMINATION.  271 

upon  the  top  of  the  pole,  the  arc  lamp  hanging  from  an 
insulated  hook  within  the  hood.  The  obvious  objec- 
tion to  pole-top  fixtures,  whether  for  arc  lights  or  for 
gas  lamps,  is  that  the  light  must  be  on  the  curb,  and 
sometimes  does  not  light  the  street  properly.  For  open 
spaces  where  the  pole  can  be  out  from  the  curb  the  pole- 
tops  work  well  and  may  be  freely  used. 

A  very  obvious  modification  is  the  lateral  bracket  car- 
rying the  lamp  well  clear  of  the  curb,  yet  not  so  far  out 
as  to  make  it  difficult  to  trim  the  lamp  from  the  pole 
without  lowering  it.  Fig.  104  is  a  specimen  of  this 
class,  which  carries  the  lamp  2.  ft.  out  from  the  pole.  If 
longer  than  this  the  lamp  should  be  supported  by  a  rope 
and  lowered  for  trimming.  Such  brackets  in  various 
forms  have  been  in  use  for  a  long  timei  and  a  neat  iron 
pole  and  bracket  dating  back  some  seventy-five  years  is 
shown  in  Fig.  105.  This  is  fitted  with  a  pulley  and  cord 
for  lowering  the  oil  lamp  for  filling.  It  might  be  copied 
to  advantage  even  now. 

A  somewhat  analogous  type  of  bracket  has  been  intro- 
duced and  very  extensively  used  by  the  Boston  Electric 
Light  Company.  It  is  a  hollow  casting,  fitted  to  the 
top  of  a  neat  wooden  pole,  and  permits  the  line  wires  to 
be  carried  within  it  clear  to  the  lamp  without  exposing 
them.  For  underground  service  the  pole  itself  may  be 
hollow,  thus  entirely  eliminating  exposed  wires.  The 
lamp  can  be  trimmed  easily  from  the  pole,  and  a  cut-out, 
A,  is  fitted  in  the  slight  expansion  near  the  base  of  the 
upright  part  of  the  casting.  Fig.  106  shows  this  very 
neat  and  convenient  fixture  in  outline. 

Mast-arms  are  really  modified  brackets  lengthened  so 
much  as  to  bring  the  lamps  nearly  or  quite  to  the  center 
line  of  the  street,  and  usually  arranged  to  permit  the 


272 


THE   ART    OF   ILLUMINATION. 


lamp  being  readily  lowered  to  the  street  for  trimming. 
Now  and  then  the  lamp  is  carried  on  a  trolley,  which  can 
be  pulled  in  to  the  pole  for  trimming,  but  the  preference 
is  generally  for  the  former  plan.  Fig.  107  shows  a  corn- 


Fig.  105. — Antique  Iron  Pole. 


Fig.  106. — Boston  Pole 
Fixture. 


mon  form  of  mast-arm  fitted  for  lowering  the  lamp. 
The  lamp  is  usually  carried  some  14  or  15  ft.  out  from 
the  pole,  hence  the  truss  form  becomes  necessary  to 
secure  the  proper  degree  of  strength  and  stiffness. 


EXTERIOR    ILLUMINATION.  273 

Mast-arms  furnish,  on  the  whole,  the  best  means  of 
carrying  the  light  out  over  the  street.  They  call  for  but 
a  single  pole  at  the  curb,  and  put  the  light  exactly  where 
it  is  wanted,  and  hold  it  there  steadily.  They  are  far 
from  beautiful,  however,  and  from  the  aesthetic  stand- 
point the  cross  suspension  is  generally  to  be  preferred. 
This  is  a  very  old  method  of  supporting  lights,  and  con- 


Fig.  107.— Mast  Arm. 

sists  merely  of  a  rope  stretched  across  the  street  and 
bearing  midway  a  pulley  from  which  the  lamp  is  carried. 
Fig.  108,  from  an  old  French  print  showing  street  light- 
ing in  Paris  early  in  the  eighteenth  century,  illustrates 
the  principle  as  well  as  a  more  modern  instance.  To- 
day the  rope  is  of  wire  strands  and  the  lamp  is  an  elec- 
tric arc,  but  the  rest  has  changed  little. 

In  ordinary  cases  the  cross  suspension  requires  a  pole 
set  at  the  curb  on  each  side  of  the  street,  which,  except 
at  corners  where  the  pole  lines  cross,  is  somewhat  of  an 
inconvenience.  Sometimes  a  tree  is  used  for  one  sup- 
port, but  the  practice  is  not  to  be  encouraged,  since  it  is 
both  bad  for  the  tree,  and  renders  the  lamp  rather  un- 
steady in  a  wind.  When  conditions  permit,  cross  sus- 


274 


THE   ART    OF   ILLUMINATION. 


pension,    however,   is   a   most   useful   and    unobtrusive 
method  of  carrying  the  lamps. 

In  general,  where  there  is  an  underground  distribu- 
tion, either  of  electricity  or  gas,  pole-top  and  bracket 
fixtures  are  most  useful  for  ordinary  purposes.  Fix- 
tures like  Fig.  105  lend  themselves  very  readily  to  artis- 
tic treatment  either  for  electric  lights  or  for  mantle  or 


Fig.  108. — Antique  Cross  Suspension. 

regenerative  gas  burners.  In  streets  thickly  shaded  by 
trees  recourse  must  generally  be  taken  to  mast-arms  or 
to  cross  suspensions  in  order  to  put  the  lights  where 
shadows  will  not  be  troublesome.  Sometimes  even  in- 
candescent lamps  are  carried  in  the  latter  manner, 
though  being  rather  closely  spaced  they,  like  mantle 
burners,  give  fairly  good  results  if  placed  alternately  on 
each  side  of  the  street  and  bracketed  clear  of  the  curb. 

Generally,  the  lighting  of  a  town  will  call  into  useful 
service  all  the  ordinary  types  of  fixtures,  and  an  attempt 
to  adopt  a  single  standard  form  will  lead  to  considerable 
embarrassment  in  the  effective  lighting  of  certain 
localities. 


CHAPTER  XII. 

DECORATIVE    AND    SCENIC    ILLUMINATION. 

IN  lighting  large  spaces  either  indoors  or  out,  effective 
use  may  be  made  of  arc  lamps  as  well  as  incandescents. 
In  some  instances  fairly  good  results  are  obtained  by 
using  for  this  purpose  ordinary  open  or  enclosed  arc  lamps 
with  large  metallic  reflectors  behind  them.  They  produce 
a  powerful  and  partially  diffused  illumination  that  is 
rather  serviceable  in  many  situations,  but  is  neither  very 
uniform  nor  intensely  brilliant.  Such  places  as  piers  are 
often  thus  lighted,  the  reflectors  saving  considerable  light 
that  would  otherwise  be  thrown  in  useless  directions  and 
wasted.  Even  a  reflector  of  tin  covered  with  white 
enamel  paint  can  be  made  very  serviceable  for  this 
purpose. 

If  for  any  reason  the  white  or  bluish  white  light  of  the 
arc  is  undesirable,  the  color  can  easily  be  slightly  modified 
by  using  on  the  arc  lamp  a  globe  of  colored  glass  or  coat- 
ing a  clear  outer  globe  with  the  solution  employed  for 
coloring  incandescent  bulbs. 

These  cannot  strongly  tinge  the  light  without  greatly 
reducing  it,  since  they  color  only  in  virtue  of  absorption, 
a  red  screen,  for  instance,  giving  a  ruddy  tinge  to  the  arc 
by  cutting  off  a  large  amount  of  blue  and  green  rays. 

In  the  absence  of  electric  lamps  display  and  scenic 
illumination  is  a  rather  difficult  matter,  this  part  of  the 
art  having  been  developed  mainly  by  the  stimulus  of 
electric  lighting.  A  certain  amount  of  display  lighting 
can  be  done  by  gas  jets  with  ample  reflectors  arranged 

275 


276  THE   ART   OF   ILLUMINATION. 

much  like  those  already  shown,  but  the  results  are  not 
generally  satisfactory,  since  on  account  of  the  heat  evolved 
the  whole  apparatus  has  to  be  bulky.  Mantle  burners 
are  nearly  useless  in  this  connection,  on  account  of  their 
offensive  color.  For  brilliant  scenic  work  the  calcium 
light  is,  however,  extremely  useful,  although  its  use  of 
many  years  in  the  theater  has  now  been  almost  abandoned 
in  favor  of  electric  arcs. 

Theatrical  lighting  effects  really  form  an  art  quite  by 
itself.  It  is  quite  impossible  to  give  a  connected  account 
of  it  apart  from  an  enormous  amount  of  detail  applicable 
to  special  problems.  Broadly,  it  may  be  divided  into 
three  branches:  The  general  illumination  of  the  stage, 
scenic  illumination  of  the  stage,  and  the  illumination  per- 
taining to  tricks  and  illusions. 

The  first  mentioned  branch  differs  radically  from 
ordinary  interior  illumination  in  that  the  lights  visible 
from  the  auditorium  take  little  or  no  part  in  the  real  work. 
The  footlights,  merely  incandescents  in  front  of  enameled 
reflectors,  are  of  primary  importance,  and  the  remaining 
illumination  has  to  be  furnished  from  the  wings  and  flies. 
Contrary  to  all  usual  practice  elsewhere,  such  illumination 
must  be  nearly  or  quite  shadowless,  for  it  would  be  most 
awkward  to  have  a  massive  stage  oak  casting  the  linear 
shadow  appropriate  to  the  board  or  canvas  on  which  it  is 
painted.  Therefore,  the  general  body  of  the  lighting 
should  be  thoroughly  diffused,  as  in  illumination  from  a 
cornice  or  from  concealed  lamps  above  the  ceiling,  and  if 
for  any  reason  shadows  are  desired,  they  should  be  pro- 
duced by  auxiliary  bright  lights  introduced  for  that 
particular  purpose  and  screened  from  throwing  telltale 
shadows  where  they  are  not  wanted. 

Not  only  must  the  general  stage  lighting  be  beautifully 


DECORATIVE  ILLUMINATION.  277 

diffused,  but  it  must  be  under  perfect  control  as  to  amount. 
To  this  end  theaters  usually  have  an  elaborate  equipment 
of  rheostats,  which  can  be  thrown  in  series  with  the  lamp 
circuits,  and  these  latter  are  in  numerous  sections,  so  that 
the  light  can  be  made  to  fade  gradually  out  without  chang- 
ing its  intensity  or  its  direction  by  perceptible  degrees. 
When  alternating  currents  are  used  the  inductive  regu- 
lators can  be  made  to  accomplish  this  very  perfectly,  and 
with  an  auxiliary  storage  battery  one  can  do  equally  well 
with  continuous  current.  Without  these  a  smooth  reduc- 
tion of  illumination  is  not  easy. 

One  of  the  useful  devices  to  this  end  is  to  divide  the 
whole  body  of  lights  into  overlapping  groups.  For  ex- 
ample, if  we  imagine  100  incandescents  to  be  massed 
across  the  flies,  the  division  would  be  somewhat  as  fol- 
lows: Lamps,  i,  6,  n,  16,  etc.,  would  form  one  circuit, 
2,  7,  12,  17,  etc.,  the  second  circuit,  and  so  on,  forming 
five  groups.  Then  a  rheostat  of  moderate  size  cut  into 
circuit  with  each  group  successively  prior  to  its  ex- 
tinguishment would  enable  the  operator  to  fade  out  the 
light  by  almost  imperceptible  steps  without  altering  its 
distribution. 

Such,  or  an  equivalent  arrangement,  is  quite  necessary, 
and  should  be  capable  of  producing  a  uniform  shadowless 
illumination  of  any  required  intensity,  from  the  full  glare 
of  a  spectacle  down  to  a  light  so  faint  that  a  candle  in  the 
hand  of  one  of  the  actors  will  cast  a  flickering  shadow, 
for  the  stage  is  seldom  really  dark,  however  dark  it  may 
seem  to  the  audience. 

Whenever  the  illumination  should  have  a  definite  direc- 
tion, it  can  easily  be  given  by  special  lights  or  circuits, 
but  the  groundwork  of  the  lighting  must  be  uniform  and 
diffused. 


278  THE   ART   OF   ILLUMINATION. 

The  mainstay  of  special  lighting  effects  is  the  stage 
projector.  In  the  rough,  this  is  a  wide  angle  searchlight. 
For  the  source,  there  is  a  first-class  focusing  arc  lamp 
taking  an  amount  of  current  which  can  be  regulated  by  a 
convenient  rheostat.  The  reflector  varies  according  to 
conditions.  Sometimes  it  is  a  polished  or  enameled  me- 
tallic parabolic  mirror,  sometimes  for  other  purposes  a 
wide  parabolic  wedge  giving  strong  lateral  distribution. 

Fig.  109  is  a  good  example  of  the  adjustable  projector 
lamp  with  universal  adjustments  for  height  and  position 
of  the  lamp,  and  carrying  at  the  base  a  well  ventilated 
rheostat  for  the  proper  regulation  of  the  current  at  the 
arc.  Such  lamps  are  made  to  take  a  considerable  amount 
of  current,  often  up  to  10  or  15  amperes,  and  give  a  very 
steady  and  powerful  light. 

For  the  general  purposes  of  stage  illumination  a  con- 
densed beam  is  not  required.  In  this  case  a  rack  at  the 
mouth  of  the  reflector  is  arranged  to  take  colored  screens 
of  any  shade  required.  Those  most  often  used  are  reds 
and  pale  blues.  By  means  of  such  reflector  lamps  are 
produced  most  of  the  gorgeous  spectacular  stage  effects, 
although  in  some  cases  regular  stereopticon  lanterns  ar- 
ranged with  the  dissolving  view  apparatus  and  fitted  with 
colored  screens  are  employed  with  admirable  results, 
particularly  in.  producing  a  very  concentrated  beam. 

No  general  directions  can  be  given  for  the  amount  of 
illumination  required  for  this  theatrical  work,  for  the 
obvious  reason  that  each  stage  setting  has  its  own  special 
requirements,  which  cannot  be  predicted.  Roughly,  the 
stage  may  require  at  times  fully  as  much  light  as  the 
auditorium  proper.  Considering  the  fact  that  the  lamps 
must  for  the  most  part  be  out  of  sight  of  the  audience  and 
in  rather  disadvantageous  position,  it  is  safe  to  say  that 


DECORATIVE  ILLUMINATION. 


279 


a  maximum  illumination  of  not  less  than  i  candle-power 
per  square  foot  should  be  provided  for,  aside  from  re- 
flector lamps  and  the  like.  Most  or  all  of  this  should  be 


Fig.  109. — Projector  Lamp 


from  incandescents,  or  gas  jets,  where  electric  lights  are 
not  available,  for  the  more  powerful  single  radiants 
dominate  the  illumination  too  strongly,  unless  used  with 
great  caution. 


280  THE    ART    OF    ILLUMINATION. 

The  more  specialized  part  of  scenic  illumination  which 
has  to  do  with  local  illusions  is  even  less  easy  to  reduce  to 
general  principles.  It  is  part  of  the  art  of  the  stage 
manager  and  his  assistants.  Since  electric  lighting  has 
become  general,  the  range  of  such  work  has  been  enor- 
mously widened.  Stage  lightning,  which  used  to  be  pro- 
duced by  a  prodigious  flash  of  lycopodium  powder  blown 
across  a  gas  jet,  is  now  beautifully  given  by  the  moment- 
ary flash  of  a  powerful  arc. 

Touches  like  Fafner's  gaudy  eyes  and  the  forging  of 
the  sword  in  "  Siegfried  "  are  due  to  the  skill  of  the  stage 
electrician,  and  would  have  been  quite  impracticable  a 
quarter  century  ago.  A  great  deal  of  temporary  work 
has  to  be  done  for  any  important  performance,  and  much 
intelligent  skill  is  required  on  the  part  of  the  operators, 
who  sometimes  have  to  follow  a  rapidly  moving  object 
about  the  stage  with  the  beam  from  a  projector,  when  a 
single  slip  would — and  sometimes  does — destroy  the  illu- 
sion and  provoke  unseemly  merriment.  It  is  almost  need- 
less to  say  that  in  stage  illusions  much  depends  on  the 
arrangement  of  the  background. 

Another  and  very  important  branch  of  scenic  illumina- 
tion is  the  decorative  lighting  of  large  buildings  and  public 
places.  The  illumination  proper  in  such  cases  has  been 
already  discussed,  but  the  intelligent  use  of  lights  to  bring 
out  the  full  value  of  their  architectural  characteristics  at 
night  is  quite  another  matter.  Even  so  apparently  simple 
a  problem  as  the  adequate  illumination  of  a  single  monu- 
ment requires  considerable  skill  and  care,  and  without 
these  almost  inevitably  fails  of  producing  the  proper 
results.  And  when  a  great  public  building  is  concerned 
the  task  becomes  far  more  difficult. 

As  a  simple  example  of  scenic  illumination  of  this 


DECORATIVE  ILLUMINATION.  2*1 

general  type  let  us  take  an  assumed  case  and  see  what  can 
be  done  with  it.  We  will  suppose  the  subject  of  our 
study  to  be  a  soldier's  monument,  such  as  may  be  found  in 
scores  or  hundreds  of  American  cities.  It  will  generally 
be  a  shaft  of  marble  or  granite,  surmounted  by  a  figure  or 
group  in  bronze,  and  with  symbolic  panels  in  bas-relief 
about  the  base.  Now,  the  wisest  course  to  pursue  is  to 
let  the  kindly  shades  of  night  cover  the  whole  affair,  but 
sometimes  the  monument  is  really  fine,  and  so  situated  that 
it  can  be  appropriately  brought  into  relief  by  suitable 
illumination,  or  the  citizens  insist  upon  lighting  it,  and  the 
attempt  has  to  be  made. 

The  broad  rule  that  governs  every  such  case  is 
that  it  is  both  impossible  and  useless  to  attempt  to  simulate 
daylight.  Full  sunlight  brings  out  details  and  produces 
effects  that  art  cannot  duplicate,  so  it  is  advisable  to  attack 
the  problem  along  quite  another  line. 

The  chief  difficulty  of  the  task  lies  in  the  fact  that 
bronze  lights  up  very  badly,  particularly  after  it  has 
acquired  the  fine  patina  given  by  age  or  skillful  chemical 
treatment.  Reflecting  little  light,  it  is  very  hard  to  bring 
into  proper  relief,  and  the  usual  result  of  attempting  it  is 
to  bring-  the  funereal  shaft  into  great  prominence,  and  to 
leave  the  figures  almost  imperceptible  in  the  general 
gloom. 

Lights  placed  upon  the  pedestal  or  shaft  almost  inevi- 
tably fail  of  reaching  any  useful  result,  by  reason  of 
throwing  their  light  too  sharply  upwards.  The  angle 
of  the  illumination  with  the  vertical  should  be  at  least  45 
degrees,  to  obtain  even  a  moderately  good  effect,  and  this 
is  very  rarely  attainable.  Arc  lights  placed  on  pole  tops 
about  the  monument  are  sometimes  tried,  but  since  from 
every  direction  of  view  some  of  them  must  be  visible  to 


2S2  THE   ART   OF   ILLUMINATION. 

the  observer  who  is  trying  to  see  the  object  which  they 
are  supposed  to  illuminate,  their  glare  quite  defeats  their 
main  purpose. 

Lights  around  the  base  may  be  able  to  illuminate  the 
pediment  properly,  but  they  should  be  enough  below  the 
general  line  of  vision  to  be  pretty  well  out  of  the  field  of 
view. 

About  the  only  way  of  getting  any  effective  illumina- 
tion at  the  top  of  the  monument  is  to  use  focusing  lamps 
with  projectors,  something  after  the  arrangement  shown 
in  Fig.  109.  Three  of  four  of  these  mounted  symmet- 
rically about  the  object  to  be  lighted  at  any  convenient 
distance  will  come  as  near  an  effective  illumination  as 
one  may  expect  to  get.  Their  beams  should  be  inclined 
upwards  enough  to  keep  them  effectively  out  of  the  field 
of  vision,  and  the  rest  of  the  monument,  if  of  light  stone, 
should  be  left  to  itself.  A  figure  wholly  light  in  tone  can 
be  very  beautifully  illuminated  by  such  means,  as  witness 
the  fine  colossal  figure  of  Liberty  at  the  Columbian  Ex- 
position, but  if  of  bronze  or  similar  dark  material,  e.  g., 
the  great  Bartholdi  Liberty,  adequate  illumination  is  both 
very  difficult,  and  if  successful,  decidedly  expensive. 

The  problem  of  effective  illumination  is  still  further 
complicated  when  the  object  is  a  large  building  or  group 
of  buildings.  The  arcs  with  reflectors,  which  may  be  so 
well  utilized  for  illuminating  a  comparatively  small  ob- 
ject, become  almost  useless  on  a  large  scale,  owing  to  the 
total  impracticability  of  furnishing  from  suitable  direc- 
tions enough  light  for  the  purpose.  The  lighting  of  a 
single  monument  may  be  regarded  as  a  special  case  of  the 
illumination  often  used  on  the  stage,  but  architectural 
illumination  is  a  matter  very  different  from  both  this  and 
from  the  illumination  of  large  open  spaces  purely  for 


DECORATIVE  ILLUMINATION.  283 

utilitarian  purposes.  It  has  for  its  object  the  display,  in 
the  most  artistic  manner,  of  the  architectural  values  of 
great  buildings  and  their  surroundings.  It  is  essentially 
decorative  rather  than  utilitarian,  and  the  methods  must 
be  governed  by  the  effect  desired  to  be  produced  rather 
than  by  considerations  of  rigid  economy. 

Such  work  must  be  done  in  connection  with  great  ex- 
positions, important  public  places,  and  sometimes  in  a 
temporary  manner  for  great  civic  functions.  The  object 
to  be  attained  is  no  longer  solely  the  illumination  of  a 
plane  near  the  ground,  but  the  bringing  into  splendid 
prominence  of  architectural  features  which  would  other- 
wise be  lost  in  the  darkness. 

The  first  fundamental  rule  in  this  class  of  work  is  to 
abandon  any  attempt  to  simulate  daylight,  and  after 
providing  for  adequate  illumination  of  the  ground  by 
means  which  shall  not  interfere  with  higher  planes  of 
illumination,  to  sketch  in  light  the  principal  effects  of  the 
scene  before  the  eye. 

Illumination  of  a  great  mass  of  buildings  by  reflected 
light  is  out  of  the  question.  If  of  dark  material,  it  is  a 
sheer  impossibility,  and  even  if  the  buildings  be  white,  the 
shadow  values  of  the  daylight  cannot  be  successfully 
imitated  by  radiants  placed  near  the  objects,  which  will 
therefore  look  either  white  and  flat  or  mottled  with  petty 
shadows  which  melt  in  the  distance  into  a  muddy  gray. 

The  configuration  of  the  lights  to  be  used  in  the  lumi- 
nous sketch  that  seems  needful  for  the  best  artistic  results 
may  be  roughly  determined  by  making  by  daylight,  or 
better,  near  sunset,  a  rough,  clear,  line  drawing  of  the 
scene  to  be  illuminated  from  a  rather  distant  viewpoint, 
the  further  as  the  scale  of  the  work  increases.  Then  the 
distribution  of  lights  following  the  principal  points  and 


284 


THE  ART   OF  ILLUMINATION. 


outlines  of  this  drawing  will  give  the  main  effects  that  one 
wishes  to  produce.  The  sketch  may  be  filled  up  by  adding 
necessary  details  not  too  brightly,  and  the  ground  illumi- 
nation must  be  such  as  not  to  interfere  with  this  general 
arrangement.  Reflected  light  from  the  radiants  thus  dis- 


Fig.  no. — Illumination  of  the  Eiffel  Tower. 

tributed  plays  a  useful  part  in  adding  to  the  general 
brilliancy  of  the  effect  without  marring  its  artistic  unity. 

This  was  the  principle  applied  in  lighting  the  Eiffel 
Tower  at  the  Paris  Exposition  of  1900,  and  the  result, 
shown  in  Fig.  no,  is  most  striking.  The  treatment  must 
depend  somewhat  on  the  distance  from  which  the  general 
view  is  to  be  taken.  The  Eiffel  Tower  demanded,  from 
its  immense  height  looming  against  the  sky,  a  simpler  and 
more  sketchy  treatment  than  would  have  been  advisable 
in  a  smaller  structure  generally  viewed  at  comparatively 


DECORATIVE  ILLUMINATION. 


285 


short  range.  Minute  detail  is  lost  at  a  distance  in  the 
general  glitter,  so  that  only  broad  treatment  remains 
practicable. 

The  result  of  this  general  method  has  never  been  so 
magnificently  shown  as  in  the  lighting  of  the  Pan- Ameri- 
can Exposition  recently  held  at  Buffalo.  This  was 


Fig.  in.— The  Electric  Tower  at  Buffalo. 

planned  by  Mr.  Luther  Stieringer,  a  past  master  in  the  art 
of  decorative  lighting — in  fact,  one  of  the  builders  of  the 
art  itself.  Two  buildings  at  this  Exposition  show  with 
beautiful  distinctness  the  artistic  value  of  the  sketching 
principle  just  indicated.  One,  the  great  Electric  Tower, 
400  ft.  high,  shown  in  Fig.  1 1 1,  is  a  perfect  example  of  the 
application  of  the  principles  just  laid  down.  This  tower 
is  the  dominating  note  of  the  whole  scheme  of  illumina- 


286 


THE   ART   OF   ILLUMINATION. 


tion,  and  it  is  therefore  brought  to  an  intensity  greater 
than  would  be  called  for  were  it  considered  by  itself. 
Even  this  characteristic  would  be  indicated  by  a  line  sketch 
of  the  whole  Exposition  in  grand  perspective. 

The  treatment  of  a  less  important  building  is  admirably 
shown  by  the  other,  the  Temple  of  Music,  Fig.  112.  It 
is  strikingly  beautiful,  yet  perhaps  might  have  been 


Fig.  112. — The  Temple  of  Music  at  Buffalo. 

improved  by  indicating  some  of  the  vertical  lines  in  the 
lower  part  of  the  dome.  A  feature  worth  mentioning  in 
the  Electric  Tower  was  the  simultaneous  turning  on  of  all 
the  lights  and  their  gradual  increase  to  normal  brilliancy 
by  the  use  of  a  huge  water  rheostat. 

In  this  method  of  illumination  powerful  radiants  are 
both  needless  and  harmful,  since  they  interfere  with  the 


DECORATIVE  ILLUMINATION.  287 

freedom  of  the  sketching  and  blur  the  effect  by  masses  of 
reflected  light.  If  used  at  all  in  the  architectural  work, 
they  should  be  used  very  sparingly,  and  Mr.  Stieringer 
very  wisely  used  the  8-cp  incandescent  lamp  as  his  unit  in 
the  great  work  at  Buffalo.  Number  of  lights,  and  power 
of  free  sketching  with  them,  is  what  is  wanted,  and  for 
this  an  8-cp  lamp  is  quite  as  effective  and  far  more 
economical  than  one  of  higher  power. 

Arcs  must  not  be  allowed  to  intrude  themselves  on  the 
effects  thus  produced,  following  the  principles  long  ago 
laid  down  in  this  volume.  When  used,  as  they  may  some- 
times be,  for  ground  or  interior  illumination,  they  should 
be  so  effectively  guarded  by  opal  globes  that  there  shall 
not  be  a  violent  contrast  in  brilliancy  between  the  various 
planes  of  illumination. 

At  Buffalo  Mr.  Stieringer  dropped  the  arc  altogether, 
save  in  certain  features  of  display  lighting,  like  the  illumi- 
nation of  fountains  and  cascades  by  reflectors,  and  pro- 
duced the  ground  lighting  by  clusters  of  incandescents. 
The  real  question,  however,  is  not  so  much  the  choice  of 
one  or  another  source  of  light  as  the  preservation  of  a 
uniform  or  skillfully  graded  tone  of  brilliancy  in  the 
general  illumination.  This  is  most  easily  secured  by  the 
use  of  incandescents  alone,  although  there  certainly  are 
cases  in  which  arcs  could  be  used  with  admirable  results 
as  part  of  the  general  scheme.  One  difficulty  with  the 
use  of  incandescents  heavily  massed  near  the  ground  is 
the  certainty  of  a  number  of  them  being  burned  out  every 
evening,  producing  unsightly  gaps  in  the  symmetry  of 
the  display.  Such  failures  are  far  less  conspicuous  at  a 
distance,  when  the  lights  melt  together.  The  general 
effect  produced  may  be  greatly  modified  by  varying  the 
number  and  intensity  of  the  lights  used.  Small  luminous 


288  THE   ART   OF   ILLUMINATION. 

units  not  too  thickly  crowded  give  a  transparency,  an 
airy,  unsubstantial  appearance,  that  is  lost  when  the 
radiants  are  so  powerful  or  so  numerous  as  to  render 
much  of  the  structure  visible  by  reflected  light. 

The  principles  of  architectural  illumination  have  been 
well  understood  and  skillfully  acted  upon,  though  perhaps 
not  definitely  formulated,  for  many  years.  Before  the 
introduction  of  electric  light  reliance  had  to  be  placed  on 
gas  jets,  lamps,  and  even  candles  for  such  work,  and  there 
is  no  doubt  that  very  beautiful  effects  were  produced, 
although  at  great  cost  of  labor,  and  only  temporarily. 
The  nature  of  the  radiants  was  such  as  almost  to  preclude 
the  possibility  of  overdoing  the  illumination,  and  only 
with  the  advent  of  electric  lights  has  there  developed  a 
strong  temptation  to  try  for  daylight  effects,  always  a 
failure  from  the  artistic  standpoint. 

The  absolute  number  of  lights  required  to  produce  cer- 
tain effects  is  more  a  matter  of  judgment  than  of  calcula- 
tion. If  a  row  of  radiants  is  intended  to  melt  into  a  line 
of  light,  of  course  far  more  lamps  are  needed  than  if  one 
merely  desires  a  row  of  star-like  points.  Both  arrange- 
ments may  be  advantageously  used  even  on  the  same 
building.  The  ordinary  8  or  i6-cp  lamps  melt  into  a 
practically  continuous  line  at  500  to  800  times  the  distance 
between  lamps,  so  that  if,  as  on  high  buildings,  they  are 
normally  to  be  viewed  from  a  considerable  distance,  they 
may  be  rather  widely  spaced,  while  near  the  ground  they 
may  well  be  more  closely  spaced.  A  little  tact  will  enable 
a  certain  perspective  effect  to  be  attained  if  desired. 

The  use  of  illumination  by  reflected  light  cannot  well 
be  combined  with  any  other  method,  except  as  the  lights 
used  for  the  illumination  may  give  enough  surface  reflec- 
tion to  enhance  the  general  brilliancy.  Therefore  the 


DECORATIVE  ILLUMINATION.  289 

beams  from  reflector  arcs  must  be  kept  away  from  reflect- 
ing surfaces  which  are  to  be  sketched  out  in  lines  of  light. 

Colored  light  can  be  effectively  used  with  reflector  arcs, 
on  white  surfaces,  on  cascades,  in  fountains,  and  the  like, 
but  is  seldom  successful  when  tried  with  incandescent 
lamps,  save  on  a  very  small  scale.  The  difficulty  lies  in 
the  dimness  of  colored  bulbs  and  the  failure  of  attempts 
to  get  delicate  tints  in  this  way.  Colored  glass  bulbs  are 
expensive,  and  coated  bulbs  accumulate  dust  and  are 
seldom  weather  proof. 

Much  decorative  lighting  is  for  temporary  purposes, 
but  with  the  present  facilities  for  obtaining  current  and 
the  temporary  mountings  that  can  readily  be  obtained,  the 
work  is  comparatively  easy. 

Special  receptacles  for  signs  and  decorative  designs  are 
now  made  in  convenient  form  for  quickly  putting  together, 
and  enable  temporary  work  for  special  occasions  to  be 
very  easily  done.  Fig.  113  shows  one  useful  form  of 
mounting  device,  in  which  the  weather-proof  receptacles 
can  be  quickly  strung  together  with  clamps  and  held 
neatly  spaced  in  any  way  desirable.  For  decorative  work 
on  a  considerable  scale  the  retaining  clamps  would,  of 
course,  be  much  longer  than  here  shown. 

There  is  a  fine  chance  for  art  in  turning  on  the  lights  in 
architectural  and  other  decorative  work.  The  water 
rheostat,  bringing  all  the  lights  simultaneously  from  a 
dull  red  glow  to  full  brilliancy,  is  by  far  the  most  compre- 
hensive scheme  for  the  purpose.  In  the  absence  of  this, 
or  in  permanent  work  of  which  only  a  part  is  regularly 
used,  the  circuits  should  be  so  arranged  as  to  allow  a  per- 
fectly symmetrical  development  of  the  lighting  without 
throwing  on  a  very  large  current  at  any  one  time. 

In  any  and  all  decorative  work  the  illumination  must 


29o  THE   ART   OF   ILLUMINATION. 

be  subordinated  to  the  general  architectural  effect.  Sins 
against  art  in  this  respect  are  all  too  common.  Imagine, 
for  example,  a  Doric  temple  with  arc  lights  at  the  corners 
of  the  roof  and  festoons  of  red,  white,  and  blue  incandes- 
cents  hung  between  the  columns.  About  a  structure  of 
such  severe  simplicity  lights  must  be  used  with  extreme 
caution,  while  more  ornate  buildings  can  be  treated  with 
far  greater  freedom  of  decoration. 

It  requires  both  fine  artistic  instinct  and  great  technical 
skill  to  cope  adequately  with  the  problems  of  decorative 
illumination.  The  tricks  of  the  art  are  manifold,  and 
mostly  meretricious.  The  facility  with  which  electric 
currents  may  be  manipulated  is  a  continual  temptation  to 
indulge  in  the  ingenious  and  the  spectacular  without  due 
regard  for  the  unity  of  the  results.  Whirligigs,  waving 
banners,  rippling  water,  and  the  like  are  better  suited  to 
a  Coney  Island  merry-go-round  than  to  serious  attempts 
at  decoration. 

Another  class  of  work,  hardly  a  part  of  ordinary  light- 
ing, but  yet  of  considerable  interest,  is  the  use  of  lights 
purely  for  decorative  purposes  in  interiors,  in  halls  and 
auditoriums  for  special  designs,  and  as  part  of  the  decora- 
tive scheme  of  ballrooms  and  the  like.  This  is  really  a 
branch  of  the  art  due  entirely  to  electric  lighting — since 
only  by  this  means  can  it  be  rendered  fully  serviceable. 
Most  branches  of  illumination  are  in  a  measure  indepen- 
dent of  the  particular  radiants  employed.  But  the  ease 
and  safety  with  which  incandescent  lamps  can  be  installed 
renders  them  peculiarly  applicable  to  such  interior  work. 

In  operating  on  a  comparatively  large  scale,  all  sorts  of 
decorative  designs  can  be  carried  out  by  means  of  8-cp  or 
i6-cp  lamps  strung  together  in  receptacles,  in  the 
manner  of  Fig.  113,  or  otherwise  temporarily  mounted 


DECORATIVE  ILLUMINATION.  291 

for  the  purpose.  For  work  on  a  smaller  scale,  or  in  the 
preparation  of  very  elaborate  designs,  other  means  may 
be  employed. 

For  purely  decorative  purposes  the  miniature  lamps 
serve  a  very  useful  purpose.  Regular  incandescents  are 
made  down  to  6,  or  even  4,  candle-power,  but  as  has 
already  been  explained,  the  filaments  for  these  powers  at 
ordinary  voltages  must  needs  be  very  slender  and  fragile, 
and  the  lamps  are  nearly  or  quite  as  bulky  as  those  of 
ordinary  candle-power. 

Hence  for  many  uses  it  is  better  to  make  miniature 
lamps  for  connection  in  series,  each  lamp  taking  5  to  25 


Fig.  113. — Chain  of  Receptacles. 

volts  to  bring  it  to  normal  candle-power.  Imagine  a 
i6-cp,  loo-volt  lamp  filament  cut  into  four  equal  parts, 
and  each  of  these  parts  mounted  in  a  separate  small  bulb, 
and  you  have  a  clear  idea  of  the  principle  involved.  Com- 
monly the  miniature  lamps  for  circuits  of  100  to  125  volts 
are  of  5  or  6  candle-power,  and  connected  five  in  series 
across  the  ordinary  lighting  mains.  Fig.  114  gives  an 
excellent  idea  of  the  size  and  appearance  of  the  perfectly 
plain  miniature  lamp.  It  is  fitted  to  a  tiny  socket  of  the 
same  general  construction  as  the  standard  sockets  for 
ordinary  lamps,  but  taking  up  so  little  room  that  the  lamps 
can  conveniently  be  assembled  in  almost  any  desired  form. 


292  THE   ART   OF   ILLUMINATION. 

It  is  not  altogether  easy  to  manufacture  these  lamps 
so  as  to  attain  the  uniformity  necessary,  if  the  lamps  are 
to  be  run  in  series,  and  this  at  present  constitutes  a  serious 
obstacle  to  their  use  on  a  large  scale.  They  are  generally 
not  of  high  efficiency,  since  great  uniformity  and  good 
life  are  the  qualities  most  important. 

They  can  be  fitted  with  tiny  ornamental  shades,  and 


Fig.  114. — Miniature  Incandescent  Lamp. 

may  be  obtained  of  various  shapes  and  colors,  so  that  very 
elaborate  decorative  designs  can  be  built  up  of  them.  In 
indoor  work  colored  lamps  may  be  freely  used,  and  are 
capable  of  producing  some  very  beautiful  effects,  but  the 
plain  or  ordinary  frosted  lamps  are  most  generally  used. 

Owing  to  the  small  size  of  the  sockets  and  fittings,  the 
miniature  lamps  can  be  packed  so  closely  as  to  produce 
the  effect  of  an  almost  uniform  line  of  light  at  com- 
paratively small  distances,  so  that  most  ornate  schemes 
of  ornamental  illumination  can  be  carried  out  by  their  aid. 
They  are  also  very  useful  in  building  up  small  illuminated 
signs. 


DECORATIVE  ILLUMINATION.  *93 


Lamps  of  special  sizes  and  shapes,  from  a  tiny 
bulb,  hardly  bigger  than  a  large  pea,  to  the  candle  -shaped 
lamp  of  5  or  6  candle-power,  are  sometimes  used  with  good 
effect  in  interior  decoration.  Figs.  115,  116,  117,  and 
118  show  some  of  the  commoner  shapes  used  for  such 
purposes.  When  a  regular  electrical  supply  is  not  avail- 
able, these  little  lamps  can  be  obtained  for  very  moderate 
voltages,  say,  from  5  to  10  volts,  and  can  be  run  in 
parallel  from  storage  cells,  or  even  from  primary  batteries, 
for  temporary  use. 

Such  small  lamps  are  sometimes  used  in  the  table 
decorations  for  banquets,  and  for  kindred  purposes.  By 
their  aid  surprising  and  beautiful  effects  are  attainable, 
which  would  be  quite  impossible  with  any  flame  illu- 
minant.  But  they  must  be  cautiously  used,  for  their  very 
facility  tends  to  encourage  their  employment  in  effects 
more  bizarre  than  artistic. 

It  is  well,  too,  to  add  a  word  of  caution  as  regards  the 
possible  danger  from  fire.  It  is  so  easy  to  wire  for  incan- 
descents  that,  particularly  when  using  miniature  lamps, 
there  is  a  natural  tendency  to  rush  the  work  at  the  expense 
of  safety.  Lamps  in  series  on  a  i  zo-volt  circuit  are  quite 
capable  of  dangerous  results  if  anything  goes  wrong,  and 
even  the  battery  lamps  are  not  absolutely  safe  in  the 
presence  of  inflammable  material. 

It  should  therefore  be  an  invariable  rule  not  to  install 
a  temporary  decorative  circuit  without  the  same  attention 
to  detail  that  would  be  exercised  in  a  temporary  circuit 
of  the  ordinary  incandescents.  The  same  precautions  are 
not  always  necessary,  but  all  the  wiring  should  be  care- 
fully done,  joints  should  be  fully  protected,  and,  particu- 
larly, lamps  should  be  kept  out  of  contact  with  inflam- 
mable material. 


294 


THE   ART    OF   ILLUMINATION. 


The  incandescent  lamp  is  often  commended  as  produc- 
ing little  heat,  and,  in  fact,  as  compared  with  other  illumi- 
nants,  its  heating  power  is  small.  But  a  vessel  of  water 
can  be  boiled  by  plunging  an  ordinary  i6-cp  lamp  in  it 
nearly  up  to  the  socket,  and  cloth  wrapped  about  such  a 
lamp  will  infallibly  be  ignited  within  a  comparatively 


Fig.  115.  Fig.  116.  Fig.  117. 

Various  Forms  of  Miniature  Lamps. 


Fig.  1 1 8. 


short  time.  The  fact  that  the  cloth  does  not  burst  into 
flame  in  a  few  minutes  does  not  indicate  safety,  for  time 
is  an  important  element  in  ignition,  and  even  an  over- 
heated steam  pipe  is  capable  of  setting  a  fire,  low  as  its 
temperature  is.  A  good  many  fires  have  been  started  in 
shop  windows  by  hanging  fabrics  too  near  to  incandes- 


DECORATIVE  ILLUMINATION.  295 

cent  lamps,  and  even  the  miniature  lamps  are  quite 
capable  of  similar  mischief  if  in  contact  with  anything 
easily  inflamed.  No  illuminant  has  so  high  an  efficiency 
that  it  produces  a  negligible  amount  of  heat  from  the 
standpoint  of  fire  risk. 

Special  cable  is  now  made  to  which  lights  can  be  at- 
tached with  great  facility,  and  by  this  means  temporary 
work  may  be  quickly  and  safely  done. 

In  ordinary  domestic  illumination  miniature  lamps  have 
very  little  place.  Nothing  is  to  be  saved  by  using  them 
so  long  as  they  must  be  used  in  series  at  ordinary  voltages. 
Now  and  then  a  4  or  6-cp  lamp  may  be  useful  as  a  night 
lamp,  but  it  is  better  to  use  an  ordinary  lamp  of  moderate 
efficiency  than  to  try  miniature  lamps.  Sometimes,  how- 
ever, a  circuit  of  miniature  lamps  may  be  installed  for  a 
dining  room  or  a  ballroom  with  excellent  artistic  results. 
In  such  cases  it  is  better  to  use  ground  than  plain  lamps, 
and,  as  a  rule,  colored  lamps  should  be  eschewed,  on 
account  of  the  impossibility  of  getting  delicate  tints  to 
show  effectively. 

Temporary  decorative  circuits  may,  however,  be  very 
useful  in  domestic  illumination  for  fetes  and  the  like,  in 
which  case  delicately  colored  ornamental  shades  can  be 
applied  or  the  lamps  may  be  used  in  Japanese  lanterns. 
Any  country  house  fitted  for  electric  lights  can  be 
be  temporarily  wired  for  such  purposes  rather  easily,  and 
out-of-door  temporary  wiring  can  be  installed  without 
the  rigid  precautions  necessary  indoors. 

In  all  decorative  lighting  it  is  important  to  recognize  the 
fact  that  illumination  is  a  means  to  an  artistic  end,  and  not 
of  itself  the  primary  object.  One  is,  in  these  days  of 
electric  lighting,  far  more  likely  to  err  by  providing  too 
much  light  than  by  failing  to  supply  enough. 


296  THE   ART   OF   ILLUMINATION. 

Great  brilliancy  is  far  less  important  than  good  distribu- 
tion and  freedom  from  glare.  It  is  highly  probable,  for 
instance,  that  the  effect  of  the  illumination  of  the  Electric 
Tower  at  the  Pan- American  Exposition  would  have  been 
seriously  injured  by  the  substitution  of  32-cp  lamps  for 
the  8-cp  actually  used,  and  it  is  absolutely  certain  that  a 
dozen  arc  lights  injudiciously  placed  would  have  detracted 
greatly  from  the  harmonious  result. 

In  interior  illumination  the  same  rule  holds  true.  By 
the  reckless  use  of  brilliant  radiants  one  can  key  the  vision 
up  to  a  point  where  its  power  of  appreciating  values  in 
illumination  is  almost  entirely  lost.  In  decorative  light- 
ing great  care  must  be  used  not  to  approach  this  point,  to 
leave  the  relief  afforded  by  light  and  shade,  and  to  realize 
the  perspective  in  the  details  of  the  illumination. 

In  the  absence  of  a  foreground  one's  judgment  of  dis- 
tances is  completely  upset,  as  witness  the  great  difficulty 
experienced  in  estimating  distances  correctly  over  water 
on  the  one  hand  or  in  a  thin  fog  on  the  other.  In  scenic 
illumination  the  distribution  of  luminous  values  can  be 
utilized  with  great  facility  for  producing  illusions  of 
distance,  giving  at  will  the  effect  of  startling  flatness  or  of 
interminable  vistas.  In  stage  illumination  such  devices 
are  now  and  then  used  to  heighten  the  effects,  although 
other  exigencies  often  interfere  with  the  proper  develop- 
ment of  the  scheme. 

The  commonest  cause  of  failure  in  proper  illumination 
is  thrusting  a  brilliant  light  between  the  spectator  and  the 
object  to  be  viewed,  with  the  inevitable  result  of  losing 
detail  and  hurting  the  eyes.  Brilliant  diffused  light  is  in 
this  particular  only  less  objectionable  than  direct  light, 
and  both  should  be  assiduously  avoided. 

It  must  not  be  supposed  that  decorative  lighting  must 


DECORATIVE  ILLUMINATION.  297 

necessarily  be  electric,  since  very  beautiful  results  were 
attained  before  electric  light  was  heard  of,  but  electric 
lighting  is  unquestionably  the  most  facile  means  of  secur- 
ing artistic  results  on  a  large  scale. 

A  special  department  of  lighting,  peculiar  to  the  electric 
branch  of  the  art,  is  the  use  of  the  searchlight  for  scenic 
or  utilitarian  purposes.  The  searchlight  is  now  a  familiar 
object,  consisting  of  a  very  powerful  arc  light,  taking 


Fig.  119. — Searchlight  Lamp. 

from  20  to  50  or  more  amperes,  kept  steadily  by  auto- 
matic focussing  apparatus  in  the  focus  of  a  parabolic 
mirror,  sometimes  with  an  auxiliary  lens  system.  The 
material  of  the  mirror  is  most  often  silvered  glass,  unless 


298  THE   ART   OF   ILLUMINATION. 

the  parabolic  surface  is  very  deep,  when  silvered  metal  is 
generally  employed. 

As  the  purpose  of  the  searchlight  is  to  give  a  parallel 
beam  of  light,  the  carbons  between  which  the  arc  is 
formed  are  not  in  line,  but  staggered,  as  shown  in  Fig.  1 19 
so  that  the  crater  of  the  arc  points  obliquely  backward, 
and  the  carbons  are  tilted  so  that  this  crater  faces  fairly 
the  apex  of  the  mirror  instead  of  its  aperture.  An  opaque 
disk  between  the  arc  and  the  mirror  aperture  cuts  off  all 
stray  direct  light,  so  that  all  the  light  sent  out  is  delivered 
from  the  mirror  in  a  nearly  parallel  beam.  The  whole 
affair  is  mounted  in  a  case  having  rotation  about  a  hori- 
zontal and  a  vertical  axis,  forming  the  familiar  device 
shown  as  a  whole  in  Fig.  120. 

The  mirror  aperture  may  vary  from  a  foot  or  so  up  to 
4  or  5  ft.,  the  searchlights  most  often  used  having  from 
2  to  3  ft.  of  aperture.  The  most  perfect  results  are  given 
by  using  a  rather  shallow  parabolic  mirror  of  silvered 
glass,  which  can  be  given  a  better  and  more  permanent 
figure  than  a  deep  metallic  mirror,  and  hence  gives  a 
beam  more  accurately  parallel. 

The  searchlight,  when  properly  constructed,  will  throw 
a  dazzling  beam  many  miles  on  a  clear  night,  but  in  foggy 
weather,  or  even  in  a  comparatively  thin  haze,  its  field  of 
usefulness  is  greatly  limited.  It  is  of  only  casual  use  in 
ordinary  forms  of  illumination,  and  its  chief  legitimate 
use  is  the  illumination  of  special  objects,  in  the  manner 
already  described  in  connection  with  the  simpler  reflector 
arcs.  It  is  often  abused  by  its  application  to  advertising, 
and  to  a  glaring  and  offensive  simulation  of  daylight  in 
places  that  have  no  need  of  it. 

It  is  of  considerable  military  and  naval  value,  serving 
to  detect  movements  of  an  enemy's  troops  or  to  pick  up 


DECORATIVE  ILLUMINATION. 


299 


hostile  vessels,  and  it  is  also  of  no  small  importance  in 
military  signaling  over  long  distances.  For  this  purpose 
it  is  turned  upwards  upon  the  distant  sky,  where  its  glare 
is  visible  in  clear  weather  even  up  to  a  distance  of  forty 
or  fifty  miles.  Then,  by  deflections  of  the  beam,  or  by 
periodically  cutting  it  off  by  a  register  shutter  over  the 
front,  communication  may  be  established  by  the  regular 
Morse  or  heliographing  code,  either  openly  or  in  cipher. 


Fig.  120. — Search  Light. 

Its  use  in  this  fashion  was  especially  striking  during  the 
recent  operations  for  the  relief  of  Kimberley. 

It  is  also  a  valuable  adjunct  in  coast  defense,  particu- 
larly of  narrow  channels  and  of  mine  fields,  where  it  can 
be  used  both  to  confuse  the  hostile  pilots  and  to  make  a 
clear  target  of  hostile  ships.  But  its  range  of  effective- 
ness for  such  purposes  is  popularly  much  over-estimated. 


300  THE   ART   OF   ILLUMINATION. 

In  clear  weather  it  would  quite  certainly  pick  up  a  large 
vessel  by  the  time  it  had  come  within  effective  gun  range. 
On  torpedo  boats  and  similar  small  craft,  however, 
painted  in  neutral  tints,  as  they  are  for  war,  the  searchlight 
has  a  useful  scope  of  little  over  a  mile  in  distance,  and  in 
hazy  weather  even  less.  It  has  therefore  for  naval,  as  for 
general  purposes,  a  somewhat  circumscribed  field  of  use- 
fulness, within  which,  however,  it  is  undeniably  of  great 
value. 


CHAPTER  XIII. 

THE    ILLUMINATION    OF  THE    FUTURE. 

AT  the  present  time  the  ordinary  materials  of  illumina- 
tion are  pretty  well  understood,  and  their  proper  use  is  a 
matter  of  good  judgment  and  artistic  sense.  Illumina- 
tion is  not  a  science  with  well-defined  canons  of  what  one 
might  call  illuminative  engineering,  but  an  art  wherein  an 
indefinable  and  uncommunicable  skill  pertains  almost  as 
it  does  in  the  magic  of  the  painter. 

There  are  certain  general  rules  that  must  be  followed, 
certain  utilitarian  ends  to  be  served,  but  whether  the  re- 
sult is  brilliantly  successful  or  hopelessly  commonplace 
depends  on  the  skill  that  inspires  it.  There  must  be  in 
effective  illumination  a  constant  adaptation  of  means  to 
ends,  and  a  fine  appreciation  of  values  that  quite  defies 
description.  One  may  attack  the  problem  of  illumina- 
ting a  great  building  with  all  the  resources  of  electrical 
engineering  at  his  command,  and  score  a  garish  failure, 
or  he  may  conceivably  be  confined  to  the  meager  bounds 
of  lamps  and  candles,  and  still  triumph. 

The  general  tendency  with  the  modern  intense  radiants 
at  command  is  to  light  too  brilliantly,  to  key  the  vision 
to  so  high  a  pitch  that  it  fails  to  appreciate  the  values  of 
the  chiar-oscu/ro  on  which  the  artistic  result  depends. 

The  desideratum  in  illumination,  except  for  a  small 
group  of  scenic  effects,  is  the  possession  of  cheap  and  fairly 
powerful  radiants  of  low  intrinsic  brilliancy  capable  of 

301 


302  THE   ART   OF   ILLUMINATION. 

modification  in  delicate  color  tones.  It  is  doubtful 
whether  these  qualities  are  compatible  with  very  high 
luminous  efficiency  in  a  flame  or  incandescent  radiant. 
In  modern  gas  and  electric  lighting  the  progress  toward 
efficiency  is  in  the  direction  of  very  high  temperature, 
which  implies  high  intrinsic  brilliancy. 

Vacuum  tubes  lamps,  at  present  in  only  a  crude  experi- 
mental stage,  give  hope  of  better  things,  but  at  great  risk 
of  color  difficulties,  particularly  if  high  efficiency  is 
reached. 

Ideally,  a  gaseous  radiant,  with  nearly  its  whole  lumi- 
nous energy  concentrated  in  the  visible  spectrum,  would 
give  magnificent  efficiency,  but  it  by  no  means  follows  that 
it  would  give  a  good  light.  Sodium  vapor  meets  the  re- 
quirements just  noted  tolerably  well,  yet  there  is  no  more 
ghastly  light  than  that  given  by  a  salted  spirit  lamp. 

It  mi'ght  be  possible  to  work  with  a  mixture  of 
gases  such  as  would  give  a  light  approximately  white  to 
the  eye,  and  yet  be  very  far  from  a  practicable  illuminant, 
for  the  phenomena  of  selective  absorption  are  such,  as  we 
have  already  seen,  that  the  color  of  a  delicately  tinted 
fabric  depends  on  its  receiving  a  certain  scale  of  colors  in 
the  light  which  it  reflects.  To  the  eye  a  much  simpler 
color  scheme  is  necessary  to  reproduce  light  substantially 
white,  and  such  light  falling  on  a  colored  fabric  would  by 
no  means  necessarily  bring  out  the  tints  that  glow  by 
daylight. 

Even  the  firefly's  secret,  could  man  once  penetrate  it, 
might  not  prove  such  a  valuable  acquisition  as  it  would 
seem  at  first  thought.  To  the  eye  the  light  of  most 
species  seems  greenish,  and,  in  point  of  fact,  it  so  com- 
pletely lacks  the  full  red  and  the  violet  rays  that  its 
effect  as  an  illuminant  on  a  large  scale  would  be  most 


ILLUMINATION  OF  THE  FUTURE.       3°3 

disagreeable,  far  worse  than  an  early  Welsbach  at 
its  most  evil  stage  of  decrepitude.  We  must  not 
only  steal  the  firefly's  secret,  but  give  him  a  few  useful 
hints  on  the  theory  of  color  before  the  net  result  will  be 
satisfactory  from  the  aesthetic  standpoint.  Firefly  light 
might  do  for  a  factory,  but  it  would  find  but  a  poor  market 
as  a  general  illuminant. 

It  is  a  somewhat  difficult  matter  satisfactorily  to  define 
the  efficiency  of  an  illuminant.  Luminosity  depends,  like 
sound,  upon  the  physiological  relations  of  a  certain  form 
of  energy,  and  cannot  be  directly  reduced  to  a  mechanical 
equivalent. 

The  commonest  conception  of  the  efficiency  of  an 
illuminant  is  to  regard  it  as  defined  by  the  proportion  of 
the  total  radiant  energy  which  is  of  luminous  wave 
lengths.  From  this  point  of  view  the  efficiency  may 
approach  unity  either  by  the  absence  of  infra-red  and 
ultra-violet  rays,  in  other  words,  by  purely  selective  radia- 
tion or  by  so  great  an  increase  of  radiation  in  the  visible 
spectrum  as  to  render  the  energy  of  the  remainder  nearly 
negligible. 

In  the  former  sense  the  luminous  radiation  of  the  firefly 
is  of  perfect  efficiency;  but,  obviously,  a  purely  mono- 
chromatic light  utilizing  the  same  total  amount  of  energy 
might  give  a  vastly  better  illumination — or  a  much  worse 
one,  according  to  the  wave  length  of  the  light  in  relation 
to  its  effect  on  the  eye. 

On  the  other  hand,  an  arc  between  tiny  pencils  of  the 
material  used  for  Nernst  glowers  is  reputed  to  give,  so 
far  as  watts  per  candle-power  go,  an  efficiency  nearly  as 
good  as  can  be  claimed  for  the  firefly.  The  experiments 
in  this  case  are  perhaps  not  beyond  cavil,  but,  even  grant- 
ing their  substantial  accuracy,  it  is  perfectly  certain  that 


3o4  THE   ART    OF   ILLUMINATION. 

such  an  arc  gives  radiation  by  no  means  confined  to  the 
visible  spectrum. 

The  most  that  can  be  said  in  a  definite  way  is  that 
assuming  a  continuous  spectrum  with  its  maximum 
luminous  intensity  in  the  yellow  or  yellowish  green,  there 
seems  to  be  little  chance  of  doing  much  better  than  about 
0.2  watt  per  candle-power. 

Until  practical  illuminants  of  some  kind  can  be  worked 
at  an  efficiency  within  hailing  distance  of  this  figure,  one 
need  scarcely  worry  about  the  possibility  of  combining 
nearly  monochromatic  radiations  so  as  to  give  true  chro- 
matic values. 

At  the  present  time  only  the  most  powerful  arcs  ap- 
proach an  efficiency  of  i  watt  per  spherical  candle-power 
when  so  shaded  as  to  be  of  much  use  as  illuminants  in  the 
ordinary  sense.  Ordinary  arcs  properly  shaded  are  good 
for  2  to  3  watts  per  candle-power,  and  even  the  best 
incandescents  will  hardly  do  better  than  4  watts  per 
candle-power. 

For  everyday  work  the  thing  most  needed  is  an  efficient 
light  of  moderate  candle-power  and  moderate  intrinsic 
brilliancy  combined  with  low  cost  and  good  color.  Save 
under  special  circumstances  very  powerful  radiants  are 
disadvantageous,  particularly  if  of  great  intrinsic  bril- 
liancy. 

Casting  about  the  field,  it  certainly  appears  at  first 
glance  as  though  most  modern  radiants  had  been  de- 
veloped in  the  wrong  direction.  In  particular,  electric 
lights  have  been  steadily  pushed  in  the  direction  of 
enormous  working  temperature  and  very  great  intrinsic 
brilliancy,  gaining  greatly  in  efficiency,  of  course,  but  los- 
ing in  convenience.  What  is  most  wanted  is  not  a  light 
giving  5000  candle-power  at  0.2  watt  per  candle,  but  one 


ILLUMINATION  OF  THE  FUTURE.       3°5 

giving  5  or  10  candle-power  at  even  i  watt  per  candle. 
The  vacuum  tube  lamp  seems  at  present  to  give  the 
greatest  chance  for  revolutionary  improvements,  and  even 
this  seems  to  involve  very  serious  difficulties. 

Similarly,  in  gaslights  we  have  regenerative  and 
mantle  burners  giving  50  or  100  candle-power  at  a  very 
good  efficiency,  but  they  are  too  powerful  and  too  bright 
to  be  entirely  satisfactory,  even  were  they  open  to  no  other 
objections.  For  most  purposes  a  Welsbach  giving  15 
candle-power  on  i  cubic  foot  of  gas  per  hour  would  be 
vastly  more  useful  than  one  giving  75  candle-power  on  4 
cubic  feet  per  hour.  Of  flame  radiants  none  save  acety- 
lene marks  any  material  advance  in  recent  years  in  point 
of  easy  applicability. 

It  would  seem  that  modern  chemistry  might  achieve 
something  of  value  in  adding  to  the  materials  of  illumina- 
tion. There  is  a  group  of  occult  substances  possessing 
enormous  power  of  giving  off  radiation  akin  to  that  in- 
volved in  the  X-ray,  whatever  that  might  be.  It  is  perhaps 
not  too  much  to  hope  that  some  material  of  similar 
potency  with  respect  to  luminous  rays  may  reward  the 
pertinacious  investigator.  There  is  no  intrinsic  reason 
why  an  exaggerated  type  of  phosphorescence,  capable  cf 
storing  sunlight  at  a  high  efficiency,  may  not  in  due 
season  be  evolved.  This  would  settle  the  artificial  light- 
ing problem — unless  the  color  were  irremediably  bad — in 
a  beautifully  simple  way.  Or  it  might  be  possible  to  re- 
produce by  a  commercial  process  the  slow  oxidation  or  an- 
alogous change  responsible  for  the  glowing  of  decaying 
wood  and  of  certain  micro-organisms,  and  probably  also 
for  the  light  of  the  firefly  and  his  allies. 

Whatever  the  method,  the  aim  of  improvement  should 
be  the  production  of  efficient  lights  of  moderate  intensity 


306  THE   ART   OF   ILLUMINATION. 

and  intrinsic  brilliancy,  coupled  with  good  color,  prefera- 
bly capable  of  easy  modification. 

The  steady  tendency  as  the  art  of  illumination  has 
advanced  has  been  towards  more  and  more  complete 
subdivision  of  the  radiants,  and  the  subordination  of 
great  brilliancy  to  perfect  distribution.  One  of  the  most 
important  lessons  of  the  Pan-American  Exposition  was 
Mr.  Stieringer's  demonstration  of  the  magnificent  useful- 
ness of  8-cp  incandescent  lamps,  skillfully  installed. 

In  the  art  of  illumination  as  much  depends  on  the 
efficient  use  of  lights  as  on  the  efficiency  of  the  lights 
themselves.  A  tallow  candle,  just  where  it  ought  to  be, 
is  better  than  a  misplaced  arc  lamp,  and,  even  taking  our 
present  illuminants  with  all  their  limitations,  skill  will 
work  wonders  of  economy. 

It  is  particularly  in  the  direction  of  adroit  use  that  the 
present  path  of  progress  lies.  One  of  the  fundamental 
facts  in  practical  lighting  which  has  been  many  times 
suggested  in  these  pages,  and  which  lies  at  the  root  of 
improvements,  is  the  need  of  keeping  down  intrinsic 
brilliancy. 

The  true  criterion  of  effective  and  efficient  lighting  is 
not  simple  illumination,  which  resolves  itself  into  a  pure 
matter  of  candle  feet,  but  visual  usefulness,  which  takes 
account  of  the  physiological  factors  in  artificial  lighting. 

If  one  denotes  the  illumination  measured  in  candle  feet 
or  other  convenient  units  by  /,  then  the  visual  usefulness 
is  measured  by  the  product  /  67  where  a  is  proportional 
to  the  effective  area  of  the  iris.  This  of  course  is  con- 
stantly shifting  as  the  illumination  changes,  but,  broadly, 
it  is  an  inverse  function  of  the  intrinsic  brilliancy  of  the 
radiants  used.  The  criterion  thus  becomes  of  the  form 

*  ~~7T#y  where   B    is    the    intrinsic    brilliancy    of    the 


ILLUMINATION  OF  THE  FUTURE.       3°7 

radiant,  and  i  is  the  visual  usefulness,  or  the  effective  bril- 
liancy of  the  illumination. 

Now  as  a  matter  of  practice  this  is  important,  for  it 
indicates  that  a  badly  placed  arc  light,  for  example,  may 
actually  work  serious  injury  to  the  effective  illumination, 
and  within  reasonable  limits  one  could  fairly  go>  so  far  as 
to  say  that  the  usefulness  of  an  unmodified  radiant  varies 
inversely  with  its  intrinsic  brilliancy.  Obviously,  then, 
shading  the  radiant  may  actually  gain  useful  illumination, 
although  it  actually  loses  light,  which  in  fact  experience 
has  shown  to  be  the  case. 

As  to  the  permissible  intrinsic  brilliancy  for  ordinary 
cases  of  illumination,  exact  figures  are  from  the  nature  of 
the  case  hardly  attainable.  Yet  one  may  derive  a  pretty 
clear  idea  of  the  situation  from  the  experiments  of  Pro- 
fessor L.  Weber  given  in  the  following  table — reduced  to 
candle-power  per  square  inch. 

Horizontal  white  card  reflecting  brilliant  sunlight,       .         .     25 
White  cloud  reflecting  brilliant  sunlight,      ....       7 

Argand  burner,  .         .         .         .         .         .         .         .6.5 

Horizontal  white  card  under  a  dull  winter  sky,  .         .       0.26 

Now  the  intensity  in  the  first  named  case  is  certainly 
most  painfully  great,  and  even  those  in  the  second  and 
third  cases  are  still  great  enough  to  be  very  unpleasant 
if  fairly  in  the  field  of  view.  On  the  other  hand,  the  last 
case  evidently  is  one  in  which  the  intrinsic  brilliancy  is 
unnecessarily  low. 

Taking  all  these  things  into  consideration,  it  is  a  safe 
working  rule  to  keep  the  intrinsic  brilliancy  of  all 
radiants  within  the  Held  of  vision  below  5  cp  per  square 
inch — preferably  down  to  half  that  value. 

This  limit  affords  a  means  of  determining  the  approxi- 
mate size  of  any  diffusing  globe  or  shade,  since  evidently 


3o8  THE   ART   OF   ILLUMINATION. 

whatever  the  candle-power  of  the  light,  the  visible  diffus- 
ing surface  must  not  exceed  a  brilliancy  of  5  cp  per  square 
inch.  If,  therefore,  we  are  dealing  with  a  light  of  100 
candle-power,  that  amount  of  light  must  be  scattered  over 
and  by  at  least  twenty  square  inches  of  diffusing  surface. 
Two  conditions  enter  to  modify  the  situation  :  On  the  one 
hand,  a  certain  amount  of  the  inwardly  incident  light  is 
actually  intercepted  by  the  shade;  on  the  other  hand,  the 
diffusion  is  not  uniform,  especially  if  the  radiant  has  great 
intrinsic  brilliancy  and  the  shade  is  fairly  translucent. 
For  heavily  ground  or  fairly  dense  opal  shades  the  above 
ratio  is  not  far  from  right,  the  modifying  factors  tending 
to  offset  one  another.  Such  shades  intercept  about  one- 
third  of  the  total  light  as  a  necessary  feature  of  keeping 
the  intrinsic  brilliancy  within  bounds,  so  that  it  is  not 
unfair  to  say  that  for  most  practical  purposes  100  candle- 
power  in  a  radiant  of  really  low  intrinsic  brilliancy  is  as 
useful  as  150  candle-power  in  a  very  intense  radiant. 

Now  practically  all  our  modern  sources  of  light 
require  shading,  if  within  the  field  of  vision.  The  obvious 
moral  is  that  one  of  the  great  .economies  in  lighting  is 
centered  in  keeping  the  radiant  out  of  this  field. 

In  electric  lighting,  incandescent  lamps  at  3  watts 
per  candle  asea  ,  so  disposed  as  to  keep  clear  of  the  field 
of  vision,  are  fully  as  valuable  illuminants  as  lamps  at  2 
watts  per  candle  wrongly  installed,  so  as  to  either  dazzle 
the  eye  or  to  require  shading  to  avoid  it.  Shaded  they 
must  be  for  hygenic  reasons  whenever  visible. 

In  actual  practice  it  is  a  matter  of  great  difficulty  to 
place  lights  wholly  out  of  the  field  of  vision,  and  the  more 
brilliant  the  lights  are  the  greater  necessity  for  shading 
them.  Hence,  it  becomes  a  difficult  matter  to  treat 
modern  illuminants  without  loss  of  efficiency. 


ILLUMINATION  OF  THE  FUTURE.       s°9 

Perhaps  the  most  promising  line  of  improvement  in 
artificial  lighting,  and  the  one  from  which  most  may  be 
expected  in  the  near  future,  is  indirect  lighting  by  dif- 
fusion. A  glance  at  the  tables  in  Chapter  III.  shows  that 
with  a  good  diffusing  surface  scarcely  more  light  is  lost 
than  is  cut  off  by  proper  shading.  As  the  intrinsic  bril- 
liancy of  the  source  rises,  the  relative  importance  of 
diffusion  increases,  since  shading  to  be  effective  must  be 
denser. 

Of  diffusing  shades  only  the  holophanes  intercept  ma- 
terially less  light  than  would  be  lost  in  a  good  diffuse 
reflection,  and  even  in  this  case  the  shade  must  be  of  con- 
siderable dimensions  to  keep  the  intrinsic  brilliancy  suf- 
ficiently low.  As  compared  with  a  ground  glass  or  opal 
shade,  they  should  have  considerably  greater  total  surface 
for  a  light  of  equal  power. 

There  is  room  for  splendid  developments  in  diffuse 
lighting,  using  arcs,  Nernst  lamps,  incandescents,  Wels- 
bach  mantles,  and  acetylene.  In  this  way  such  radiants  can 
be  used  unshaded  with  the  full  advantage  of  their  great 
efficiency,  and  with  good  diffusion  from  white  or  nearly 
white  surfaces  the  net  efficiency  remains  high.  As  has 
already  been  noted,  lighting  by  diffusion  in  ordinary  dwell- 
ings, where  the  surfaces  are  not  generally  good,  requires 
a  liberal  use  of  light,  but  with  a  careful  study  of  the 
conditions  will  come  the  possibility  of  very  efficient  and 
beautiful  lighting  in  which  the  radiants  shall,  save  in  rare 
instances,  be  wholly  invisible. 

This  method  of  working,  too,  has  a  great  artistic  advan- 
tage, in  that  the  light  can  be  successfully  modified  by 
tinted  diffusing  surfaces  with  far  greater  success  than  by 
any  arrangement  of  colored  shades. 

The   latter   are   not   available   in    delicate   and   easily 


3io  THE   ART    OF   ILLUMINATION. 

graduated  shades,  while  pigments  can  be  worked  upon 
diffusing  surfaces  in  almost  any  desired  manner. 

It  thus  becomes  possible  to  use  effectively  not  only 
radiants  of  intrinsic  brilliancy  too  great  to  be  easily 
managed  by  shades,  but  those  of  naturally  objectionable 
colors.  Bad  color  is  of  course  equivalent  to  inefficiency 
in  many  instances,  since  a  considerable  amount  of  light 
must  be  cut  off  and  thrown  away  to  correct  the  color,  but 
this  can  be  done  at  as  little  loss  by  diffuse  reflection  as  by 
any  other  method. 

The  weak  point  of  lighting  by  diffusion  is  the  fact  that 
the  radiants  are  then  usually  installed  in  rather  inaccessi- 
ble places,  and  the  globes  are  likely  to  suffer  from  dust, 
unless  special  care  is  taken.  A  favorite  location  for  such 
lights  is  above  and  partly  behind  a  cornice,  a  situation  in 
itself  very  advantageous  but  difficult  to  get  at.  Ordinary 
ceiling  or  cornice  incandescent  lamps  can  be  removed  for 
cleaning  by  a  special  handler  made  for  the  purpose,  but 
lights  behind  a  cornice  must  be  reached  with  a  step 
ladder. 

Gaslights,  of  course,  cannot  be  readily  installed  in  such 
a  situation,  and  when  used  by  diffusion  must  be  screened 
like  arc  lamps. 

The  introduction  of  new  illuminants  is  very  likely  to 
effect  useful  modifications  in  our  methods  of  lighting. 
If  the  vacuum  tube  line  of  experimentation  leads  to  any- 
thing practical,  it  will  probably  provide  light  of  rather 
low  intrinsic  brilliancy,  so  that  shading  will  be  relatively 
less  important  than  it  now  is.  There  will  thus  be  a 
practical  gain  in  efficiency  even  greater  than  the  gross 
relative  efficiencies  of  the  lights  would  indicate. 

Perhaps  the  most  promising  light  of  the  class  just 
mentioned  is  the  mercury  lamp,  to  which  some  reference 


ILLUMINATION  OF  THE  FUTURE.        3™ 

has  already  been  made.  Up  to  the  present  its  very  ob- 
jectionable color  stands  in  the  way  of  its  commercial 
development,  but  if  this  fault  can  be  remedied  the  mercury 
lamp  certainly  has  a  future,  since  it  is  highly  efficient, 
and  can  be  worked  successfully  on  the  ordinary  con- 
tinuous current  constant  potential  circuits.  Most  vac- 
uum tube  schemes,  and  indeed  most  of  the  other  devices 
recently  suggested  for  securing  high  efficiency,  require 
alternating  currents,  so  that  the  mercury  lamps  would  be 
particularly  welcome  as  averting  the  need  of  an  extensive 
change  of  equipment. 

Increase  of  efficiency  in  mantle  radiants  may  in  some 
degree  be  obtained  by  the  use  of  substances  giving  more 
strongly  selective  emission  of  light  than  any  now  familiar, 
but,  aside  from  this,  efficiency  can  only  be  raised  by  forc- 
ing the  temperature. 

The  somewhat  promising  field  of  phosphorescence  is 
practically  unexplored.  A  few  interesting,  but  so  far 
futile,  experiments  have  been  tried  by  Edison  and  others 
as  a  result  of  X-ray  investigations.  The  subject  is  well 
worth  investigation,  both  from  the  electrical  and  the 
chemical  sides,  and  will  doubtless  take  its  turn  sooner  or 
later. 

Meanwhile  we  must  do  the  best  we  can  with  the 
illuminants  which  are  now  at  hand,  to  furnish  light  of 
suitable  amount  and  quality.  To  sum  up  the  suggestions 
repeatedly  made  in  these  pages,  the  commonest  failings 
in  present  methods  of  lighting  are  a  tendency  to  use  too 
brilliant  radiants  and  to  make  up  in  quantity  what  is  lack- 
ing in  quality.  More  study  of  the  practical  conditions 
of  lighting  and  less  blind  faith  in  bright  lights  would 
generally  both  improve  practical  illumination  and  tend  to 
economy. 


312  THE   ART   OF   ILLUMINATION. 

Imagine,  for  example,  an  attempt  to  light  a  billiard  room 
where  the  balls  had  been  stained  to  match  the  cloth.  Yet 
this  sort  of  thing,  on  a  less  aggravated  scale,  happens  far 
oftener  than  would  be  thought  possible.  Even  in  build- 
ings designed  to  fulfill  hygienic  conditions,  sins  against  the 
fundamental  principles  of  lighting  are  distressingly  com- 
mon. An  observing  writer  has  grimly  designated  modern 
schools,  "  Bad-eye  factories,"  and  certainly,  even  with  the 
full  advantage  of  natural  light  and  buildings  in  which 
conditions  ought  to  be  favorable,  the  results  are  frequently 
bad. 

With  artificial  light  the  task  of  proper  lighting  is  of 
increased  difficulty,  and  is  further  complicated  by  the 
sometimes  impossible  requirements  of  the  latest  fashion- 
able scheme  of  decoration.  The  best  results  can  be  at- 
tained only  by  constant  attention  to  details  and  a  keen 
perception  of  the  conditions  to  be  met. 

The  illumination  of  the  future  ought  to  mean  the  intel- 
ligent use  of  the  lights  we  now  have,  not  less  than  the 
application  of  the  lights  which  we  may  hope  in  the  full- 
ness of  time  to  obtain. 


CHAPTER  XIV. 

STANDARDS    OF    LIGHT    AND    PHOTOMETRY. 

OF  all  important  physical  constants  none  are  in  so  un- 
satisfactory a  state  as  those  pertaining  to  illumination. 
In  spite  of  the  efforts  of  several  scientific  congresses,  there 
is  no  international  convention  regarding  the  unit  of 
luminous  intensity,  nor  is  there  any  one  practical  unit  in 
general  use. 

A  standard  to  be  worthy  the  name  should  be  accurately 
reproducible  without  extreme  difficulty,  and  ought,  as 
well,  to  bear  a  fairly  simple  relation  to  other  units  which 
are  related  to  it.  Now,  a  standard  of  light  stands  quite  by 
itself  in  kind,  and  should  consist  of  some  determinate  light- 
giving  body  so  constituted  that  it  can  be  reproduced  and 
used  in  any  part  of  the  world  without  material  error. 

Unhappily,  such  a  light-giving  body  is  not  easily,  if  at 
all,  obtainable,  hence  the  present  confusion. 

The  only  attempt  yet  made  to  produce  a  really  logical 
and  scientific  unit  was  that  brought  to  the  world's  attention 
by  M.  Violle  at  the  international  electrical  congress  held 
in  Paris  in  1884.  Violle  proposed  as  the  unit  of  luminous 
intensity  the  light  emitted,  normal  to  its  surface,  by  one 
square  centimeter  of  platinum  at  its  melting  point. 

This  unit  was  one  based  on  definite  things;  was  of  very 
convenient  magnitude,  about  20  candle-power;  and  of 
good  color,  nearly  white.  But  it  is  a  very  inconvenient 
unit  to  work  with,  and  to  reproduce  accurately,  on  account 

313 


3i4  THE   ART   OF   ILLUMINATION. 

of  the  enormous  temperature  necessary  to  melt  platinum, 
the  uncertainty  introduced  as  to  its  exact  melting  point 
by  the  presence  of  trifling  impurities,  and  for  other  minor 
but  sufficient  reasons.  So  the  upshot  of  the  matter  has 
been  that  this  unit  has  been  laid  upon  the  shelf,  and  while 
much  good  came  from  the  agitation  of  the  subject,  the 
world  still  depends  on  the  curious  assortment  of  units 
sanctioned  by  more  or  less  extensive  usage. 

The  practically  and  legally  adopted  unit  of  light  in 
English-speaking  countries  is  the  so-called  standard  can- 
dle. This  illuminant  has  its  composition,  dimensions, 
weight,  and  rate  of  burning  specified  by  law,  and  can  be 
cheaply  and  easily  obtained.  It  is  a  spermaceti  candle, 
weighing  1200  grains  avoirdupois  (6  to  the  pound),  and 
burning  at  the  rate  of  120  grains  per  hour.  The  standard 
diameter  is  0.8  inch  at  the  top  and  0.9  inch  at  the  base, 
and  the  normal  height  of  the  flame  is  about  i%  inches 
over  all. 

The  rate  of  burning  may  vary  in  practice  from  about 
no  grains  to  130  grains  per  hour,  and  in  photometric 
work  the  luminous  intensity  is  assumed  to  vary  directly 
with  the  rate  of  burning.  Selected  candles  burned  under 
uniform  conditions  run  somewhat  closer  to  the  standard 
rate  of  burning  than  the  above  figures,  and  burn  with  a 
uniformity  that,  considering  their  structure,  is  remarkable, 
but  the  presence  of  the  wick,  accidental  variations  in 
manufacture,  and  numerous  minor  causes  make  the  candle 
at  best  rather  unreliable.  With  great  care  in  using  it 
may  be  coddled  into  a  degree  of  precision  of  approxi- 
mately two  or  three  per  cent. ;  but  variations  of  twice  that 
amount  are  common. 

In  France,  and  to  a  considerable  extent  in  Italy,  the 
Carcel  lamp  is  used.  This  is  a  standard  which  was 


STANDARDS  OF  LIGHT.  315 

adopted  after  the  investigations  of  Dumas  and  Regnault 
some  forty  years  since.  It  is  an  oil  lamp  of  special  con- 
struction, made  according  to  a  very  minute  specification 
as  to  dimensions,  including  the  structure  and  weight  of 
the  wick,  and  burns  refined  colza  oil,  largely  used  as  an 
illuminant  in  France.  Its  normal  consumption  of  this 
oil  is  42  grams  per  hour,  with  a  permissible  variation  of 
4  grams  per  hour  in  either  direction.  It  gives  a  rather 
yellowish  light  of  nearly  10  candle-power,  and  has  prob- 
ably about  the  same  possible  degree  of  precision  as  the 
English  candle,  though  it  should  average  a  little  better. 

In  Germany  a  standard  paraffine  candle,  made  in  pursu- 
ance of  a  most  elaborate  specification,  is  used  to  some 
extent.  It  carries  a  longer  flame  than  the  English  candle, 
two  inches  being  the  standard  height,  and  is  about  10  per 
cent,  more  powerful,  with,  in  other  respects,  much  the 
same  general  properties. 

The  standard  most  used  in  Germany,  however,  and 
often  employed  in  other  countries  for  purposes  of  refer- 
ence, is  the  so-called  Hefner  unit,  being  the  light  given 
by  the  amylacetate  lamp  introduced  by  Von  Hefner- 
Alteneck.  This  standard  lamp  is  made  from  a  uniform 
specification  as  to  dimensions,  and  has  the  great  advan- 
tage of  burning  a  perfectly  definite  chemical  compound 
easily  obtained  in  a  state  of  great  purity.  It  has  been 
very  exhaustively  investigated  at  the  Reichanstalt,  from 
which  certified  tested  standard  lamps  can  readily  be 
obtained,  and  its  performance  under  varying  conditions 
of  flame  height,  temperature,  barometric  pressure,  and  so 
forth,  has  been  carefully  studied.  Its  normal  flame  is 
40  mm.  high  and  its  intensity  is  then  about  10  per  cent, 
less  than  that  of  the  English  candle. 

Being  the  legal  standard  in  Germany,  and  widely  used 


3i6  THE   ART   OF   ILLUMINATION. 

elsewhere  on  account  of  its  steadiness  and  the  accessibility 
of  certified  examples,  the  Hefner-Alteneck  lamp  comes 
nearer  to  being  a  real  international  standard  than  any 
other.  When  used  in  strict  accordance  with  the  minute 
instructions  accompanying  each  lamp,  it  is  subject  to 
errors  less  than  half  as  great  as  those  met  with  in  standard 
candles,  and,  while  not  perfectly  steady,  is  far  steadier 
than  a  candle  or  a  Carcel  lamp.  Its  weakest  point  is  its 
color,  which  is  distinctly  reddish  orange. 

This  constitutes  a  rather  serious  objection  to  its  use 
as  a  working  standard  in  measurements  made,  for  in- 
stance, on  mantle  burners  or  incandescent  lamps.  Even 
as  a  primary  standard  its  color  and  rather  small  intensity 
form  an  obstacle  to  its  convenient  use;  but  all  in  all  it  has 
been  rather  generally  recognized  as  the  best  primary 
standard  yet  devised. 

Reproducibility  is  after  all  one  of  the  most  important 
requirements  in  a  primary  standard,  and  this  the  Hefner- 
Alteneck  lamp  possesses  in  a  very  unusual  degree. 

In  working  standards  the  most  necessary  qualities  are 
great  temporary  steadiness  and  convenience  as  to  color 
and  intensity.  These  requirements  are  far  more  easily 
met  than  that  of  exact  reproducibility,  and  in  practical 
photometry  reliable  secondary  standards  are  obtained 
with  comparative  ease. 

One  of  the  simplest  and  most  useful  is  obtained  from 
an  Argand  gas  burner,  such  as  has  already  been  described 
as  used  for  testing  purposes. 

Burned  at  a  carefully  regulated  pressure,  with  a  delicate 
meter  by  which  to  adjust  the  consumption,  and  a 
blackened  screen  to  cut  off  all  the  light  save  that  through 
a  narrow  aperture  of  definite  dimensions,  a  gas  jet  gives 
a  wonderfully  steady  light,  extremely  well  suited  to 


STANDARDS  OF  LIGHT.  317 

photometric  work.  This  arrangement  is  substantially 
that  of  the  Methven  screen,  which  is  widely  used  in 
photometry.  If  it  were  practicable  to  prepare  at  short 
notice  a  gas  of  definite  composition,  this  apparatus  might 
make  a  good  primary  standard,  but  attempts  along  this 
line  have  not  been  very  successful.  Acetylene  has  been 
suggested  for  the  purpose,  but  experience  has  shown  that 
it  is  peculiarly  subject  to  variations  in  luminous  intensity, 
and  is  worthless  as  a  standard  illuminant. 

Aside  from  the  Methven  screen,  the  most  generally  used 
secondary  standard  is  the  incandescent  lamp.  If  the 
filament  is  not  worked  at  too  high  a  temperature,  i.  e.,  at 
too  great  efficiency,  and  is  aged  by  several  hundred  hours 
of  preliminary  burning,  it  constitutes  an  admirably  re- 
liable standard. 

Burned  at  a  fixed  and  uniform  voltage,  its  intensity  can 
be  accurately  determined  by  comparison  with  a  primary 
standard,  and  remains  very  nearly  uniform,  having  a 
slight  and  definitely  ascertainable  decrement  with  time. 

In  practical  photometry  such  a  lamp  is  merely  balanced 
against  an  ordinary  aged  lamp  used  in  the  photometer  for 
testing  purposes,  remaining  itself  a  standard  of  reference. 

Several  attempts  have  been  made  at  an  incandescent 
lamp  as  a  primary  standard,  the  filament  being  of  definite 
material  and  dimensions  enclosed  in  a  globe  of  specified 
character,  and  worked  with  a  definite  amount  of  current. 

The  result  has  not  so  far  been  encouraging,  and  in  the 
absence  of  anything  better  the  Hefner-Alteneck  lamp  is 
the  main  reliance  as  a  reproducible  standard. 

At  the  time  the  Violle  standard  was  proposed  the  one- 
twentieth  part  of  it  was  tentatively  adopted  as  a  working 
unit  and  was  styled  the  bougie  decimale,  but  this  some- 
what hypothetical  unit  has  never  come  into  any  repute, 


THE   ART   OF   ILLUMINATION. 


although  its  relation  to  the  more  common  standards  has 
been  determined  with  a  fair  degree  of  precision. 

The  following  table  gives  the  relation  between  the 
several  primary  standards  here  referred  to  with  as 
much  precision  as  the  nature  of  the  case  permits,  per- 
haps rather  more,  since  one  must  admit  that  in  photometry 
the  third  significant  figure  is  of  very  dubious  value  : 


BOUGIE 

DECI- 

MALE. 

CARCEL. 

HEFNER 
UNIT. 

GERMAN 
CANDLE. 

ENGLISH 
CANDLE. 

Bougie  decimale  

Carcel                   .          

9  6 

0.6 

0.885 

O.8l5 

0.91 

German  candle  

0.104 

There  are  here  some  evident  discrepancies  which  serve 
to  mark  the  unsatisfactory  state  of  the  art,  and  to  measure 
the  uncertainties  which  exist. 

Given  a  standard,  such  as  it  may  be,  the  process  of  com- 
paring a  given  radiant  with  it  is  extremely  simple  in 
principle  and  somewhat  troublesome  and  unsatisfactory 
in  practice.  The  difficulties  come  in  part  from  the  in- 
herent difficulties  of  the  process  in  general,  and  in  part 
from  the  complications  introduced  by  variations  in  the 
color  of  the  light. 

The  Bunsen  screen,  which  in  ordinary  practice  is  the 
backbone  of  photometry,  has  already  been  in  some  meas- 
ure described,  together  with  one  of  its  simplest  applica- 
tions. The  general  principle  is  that  a  translucent  spot  in 
a  nearly  opaque  screen  of  light  texture  disappears  when 
equally  illuminated  from  each  side. 

But  for  this  to  happen  requires  that  the  screen  be  en- 
tirely symmetrical.  Light  falling  upon  it  must  be  trans- 
mitted through  and  reflected  from  the  surface  of  the 


STANDARDS  OF  LIGHT.  319 

grease-spot  in  precisely  equal  measure  irrespective  of  the 
side  of  the  spot  on  which  the  light  falls.  Jf  not,  when 
viewed  obliquely  from  one  side  the  spot  will  seem  to 
disappear  at  a  particular  point,  but  when  viewed  from  the 
other  side  this  point  will  be  shifted.  Moreover,  unless 
the  screen  be  viewed  from  the  same  angle  on  each  side, 
it  will  not  balance  at  the  same  point,  even  if  the  spot  be 


Fig.  121. — Bunsen  Photometer  Screen. 

absolutely  symmetrical  as  regards  its  two  faces.  If  this 
condition  is  fulfilled,  one  side  of  the  spot  will  generally 
accumulate  dust  a  trifle  more  freely  than  the  other,  and 
throw  things  out  of  balance  again. 

To  eliminate  as  far  as  possible  such  difficulties,  it  is 
usual  to  arrange  the  Bunsen  screen  so  that  both  sides  can 
be  observed  simultaneously,  and  from  the  same  angle. 
To  this  end  the  apparatus  is  arranged  as  in  Fig.  121. 
The  screen  marked  sc  in  the  cut  is  placed  in  a  blackened 
box  having  openings  in  the  ends  along  the  line  xy  between 
the  lights  to  be  compared,  and  a  lateral  opening  o,  in 
which  the  edge  of  the  screen  is  central.  Two  ordinary 
pieces  of  mirror,  cut  side  by  side  from  the  same  glass,  are 
set  vertically  in  the  screen  box  in  the  positions  mm',  as 


320  THE   ART   OF   ILLUMINATION. 

shown.  To  the  observer  looking  fairly  into  o  the  re- 
flected images  of  the  two  sides  of  the  screen  then  appear 
side  by  side,  and  the  slightest  change  in  the  appearance  of 
either  may  be  at  once  noted.  Sometimes  the  mirrors  are 
fitted  to  slide  out  so  that  they  may  be  interchanged  and 
another  reading  taken,  and  sometimes  the  sight  box  itself 
is  arranged  to  revolve  180  degrees  about  a  horizontal  axis 
in  the  plane  of  the  screen.  The  interior  of  the  box  must 
be  blackened  with  extreme  care  to  avoid  diffused  light. 

In  observing  with  this  sight  box  one  soon  falls  into  a 
very  uniform  habit  of  setting  the  screen  by  reference  to 


Fig.  122. — Bunsen  Photometer. 

both  its  sides,  and  can  take  wonderfully  concordant  read- 
ings. But  vision  differs  in  different  persons,  and  the 
"  personal  equation  "  in  photometric  work  is  of  consider- 
able importance. 

Aside  from  the  sight  box,  the  essential  parts  of  a 
photometer  are  a  long,  graduated  bar  along  which  the 
sight  box  can  be  slid,  suitable  supports  for  the  lights  to  be 
compared,  so  that  they  may  always  be  in  their  proper  rela- 
tion to  the  graduated  bar,  and  the  screens  already  referred 
to  for  cutting  off  stray  light.  The  elementary  arrange- 
ment of  a  Bunsen  photometer,  except  for  the  screens,  is 
very  well  shown  by  Fig.  122.  The  two  lights  are  sup- 


STANDARDS  OF  LIGHT.  321 

ported  at  known  equal  distances  from  the  ends  of  the 
graduation,  and  the  sight  box  is  then  slid  along  the  bench 
until  the  grease  spot  shows  a  balance  between  the  illumina- 
tion from  the  two  sides.  Then  the  intensities  of  the  two 
lights  are  inversely  as  the  squares  of  their  respective  dis- 
tances from  the  grease  spot. 

This  relation  assumes  that  the  lights  illuminate  their 
respective  sides  of  the  Bunsen  screen  strictly  according  to 
the  law  of  inverse  squares,  uncomplicated  by  any  sensible 
regular  or  diffused  reflection.  Right  here  is  where  the 
trouble  begins.  No  one  who  has  not  tried  it  realizes  the 
difficulty  of  eliminating  reflection.  There  must  be  no 
reflecting  surfaces  about  the  photometer,  and  it  must  be 
in  a  darkened  room  with  non-reflecting  walls,  as  far  as  it 
is  possible  to  obtain  them.  Several  coats  of  dead  black 
paint  prepared  from  lampblack  with  just  enough  thin 
shellac  to  serve  as  a  medium  answers  the  purpose  fairly 
well.  The  photometer  bench  should  allow  not  less  than 
six  feet  between  the  lights,  and  better  eight  or  ten,  A 
room  about  twelve  feet  by  six  feet  is  a  convenient  size  for 
photometric  work,  and  the  higher  the  better,  as  a  low 
room  is  apt  to  become  unpleasantly  hot  after  working  in 
it  awhile.  The  bench  should  run  along  one  side,  and  all 
the  apparatus  should  be  stowed  on  a  shelf  under  it  within 
easy  reach  of  the  hand,  for  the  room  should  be  kept  as 
dark  as  possible  to  avoid  loss  of  sensitiveness  in  the  eye. 

A  couple  of  small  heavily  shaded  incandescent  lamps, 
with  switches  in  easy  reach,  form  a  convenient  means  for 
securing  what  little  light  is  needed,  and  it  is  convenient 
also  to  have  a  tiny  miniature  lamp,  with  a  ground  bulb 
and  an  opaque  screen  to  keep  the  light  from  the  observer, 
carried  on  the  sight  box  just  above  the  pointer.  This 
lamp  should  be  furnished  with  a  mere  contact  key  on  the 


322,  THE   ART   OF   ILLUMINATION. 

carriage  of  the  sight  box,  so  that  it  can  be  momentarily 
lighted  to  read  the  graduated  scale. 

For  very  precise  work  the  Lummer-Brodhun  photo- 
metric screen  is  sometimes  used.  This  need  not  be  de- 
scribed here,  further  than  to  say  that  it  is  a  somewhat 
complicated  but  beautifully  effective  device,  rather  costly, 
and  not  as  widely  used  in  this  country  as  the  simpler 
Bunsen  screen.  Opinions  differ  widely  as  to  the  real 
relative  merits  of  these  two  devices.  In  the  writer's  judg- 
ment the  Lummer-Brodhun  screen,  when  carefully  used 
for  the  comparison  of  lights  not  differing  greatly  in  in- 
tensity or  color,  permits  a  somewhat  closer  balance  than 
the  Bunsen  screen,  but  under  ordinary  conditions  the 
latter  is  nearly  or  quite  as  effective,  and  much  easier  to  use. 

The  general  structure  of  the  photometric  apparatus 
should  be  rigid  and  substantial.  All  the  working  parts 
should  move  easily  and  smoothly,  and  all  the  accessories 
should  be  as  conveniently  placed  as  possible,  so  as  not  to 
distract  the  attention  of  the  observer  from  the  work  in 
hand. 

Attempts  are  sometimes  made  to  reduce  the  photometer 
to  a  compact,  portable  form,  that  can  be  easily  set  up  for 
testing  in  any  convenient  location.  As  a  rule,  such  porta- 
ble photometers  are  rather  unreliable.  It  is  hard  enough 
to  do  precise  photometric  work  under  the  most  favorable 
conditions,  and  in  portable  apparatus  the  tendency  is  to 
sacrifice  too  much  to  compactness.  For  certain  classes 
of  work  in  which  high  precision  is  not  necessary,  the 
portable  photometers  are  convenient,  but  they  are  not  to 
be  advised  for  general  purposes. 

Many  commercial  photometers,  both  permanent  and 
portable,  are  provided  with  scales  so  graduated  as  to  read 
candle-power  directly,  assuming  a  certain  fixed  distance 


STANDARDS  OF  LIGHT.  323 

between  the  lights  under  comparison  and  a  fixed  intensity 
of  the  standard.  It  is,  of  course,  much  easier  to  make 
photometric  tests  rapidly  with  such  a  scale,  but  it  should 
be  used  with  extreme  caution,  and  as  an  auxiliary.  When 
once  correctly  adjusted  it  is  most  convenient,  but  it  should 
be  assumed  to  be  mal-ad justed  at  the  start,  and  its  correct- 
ness carefully  verified  before  it  is  regularly  used,  and  it 
should  be  subsequently  checked  at  frequent  intervals. 
The  same  precaution  should  be  taken  with  any  other 
apparatus  graduated  for  convenience  in  arbitrary  units. 

The  holders  for  the  lights  to  be  compared  should  be 
easily  adjustable,  so  as  to  enable  the  operator  to  bring 
the  luminous  areas  into  exactly  the  right  position  with 
respect  to  the  graduated  scale.  When  incandescent  lamps 
are  under  test  it  is  convenient  to  mount  the  lamp  to  be 
tested  upon  a  rotating  spindle,  so  that  by  revolving  it  at 
the  rate  of  three  or  four  turns  a  second  the  mean  hori- 
zontal candle-power  may  be  obtained  at  a  single  reading. 
Other  sources  of  light  are  generally  also  measured  hori- 
zontally, but  in  a  single  conventional  azimuth,  and  it  is  a 
question  whether  in  the  long  run  it  is  not  better  to 
measure  incandescent  lamps  in  a  similar  fashion.  If  any 
mean  value  of  the  luminous  energy  is  to  be  considered 
important,  it  is  the  mean  spherical  rather  than  the  mean 
horizontal,  and  it  has  already  been  explained  how  by 
changing  the  shape  of  the  lamp  filament  the  distribution 
may  be  widely  altered  without  being  changed  in  amount, 
so  that  spherical  candle-power  is  really  the  significant 
thing. 

Rotators  for  incandescent  lamps  are,  however,  con- 
venient, and  particularly  so  if  arranged  so  as  to  allow  the 
axis  of  rotation  to  be  tilted  at  any  required  angle.  But 
they  require  watching,  if  accurate  work  is  desired,  since 


324  THE   ART   OF   ILLUMINATION. 

it  is  very  difficult  to  avoid  small  and  variable  losses  in 
voltage  at  the  lamp  due  to  varying  resistance  at  the 
brushes  which  convey  the  current  from  the  fixed  to  the 
rotating  part  of  the  device.  Mercury-cup  contacts  are 
somewhat  more  reliable,  but  do  not  lend  themselves  readily 
to  tilting  the  axis  of  rotation. 

Fig.  123  shows  an  excellent  typical  form  of  photometer 
intended  primarily  for  testing  incandescent  lamps,  but 
readily  adaptable  to  more  general  purposes.  It  consists 


Fig.  123.— Photometer  Bar  Complete. 


of  a  pair  of  little  standards  supported  by  cast  iron  columns, 
and  supporting  in  turn  the  lights  and  their  accessories  and 
the  pair  of  steel  shafts  extending  between  them  and  bear- 
ing the  photometer  carriage.  The  forward  bar  carries  the 
graduation.  On  the  left  is  the  carriage  for  the  standard 
lamp,  screened  in  front  and  curtained  behind,  and  on  the 
right  is  the  rotator,  similarly  screened,  for  the  lamp  to  be 
tested.  A  pair  of  sliding  screens  help  to  cut  off  extrane- 
ous light  from  the  sight  box,  and  each  lamp  is  provided 
with  a  rheostat  for  the  exact  adjustment  of  its  voltage, 
and  with  the  necessary  electrical  connections. 

In  setting  up  such  a  photometer,  even  in  a  room  painted 
dead  black,  the  screens  supplied  should  be  supplemented 
by  other  and  larger  ones  placed  nearer  the  sight  box  to  cut 
off  indirect  illumination.  It  would  also  be  advisable  to 


STANDARDS  OF  LIGHT.  325 

place  a  long  shelf  from  standard  to  standard  under  the 
photometer  bar.  This  should  be  painted  dead  black,  and 
used  to  carry  instruments  and  accessories  ready  to  the 
observer's  hand.  In  this  instance  the  distance  between 
lights  is  made  either  two  or  three  meters,  the  longer  bar 
being  preferable  for  measurements  of  precision. 

The  sight  box  is  mounted  on  trunnions,  so  as  to  be 
reversible  as  a  whole  with  respect  to  the  ends  of  the  bar. 
In  thus  reversing,  the  errors  due  to  difference  in  the  re- 
flecting mirrors  or  in  the  sides  of  the  Bunsen  screen 
proper,  as  well  as  the  personal  errors  between  the  ob- 
server's two  eyes,  are  eliminated  from  the  result.  There 
is,  however,  a  personal  error  as  between  different  observers 
that  it  not  easy  to  be  rid  of.  The  idiosyncrasies  of  the  eye 
in  photometric  work  almost  pass  understanding.  Two  ob- 
servers setting  the  Bunsen  screen  alternately  on  the  same 
lights  in  quick  succession  will  not  infrequently  obtain 
results  differing  by  nearly  10  per  cent.,  each  man's  read- 
ings, however,  being  closely  consistent.  The  same  ob- 
server will,  as  a  rule,  get  consistent  results  from  day  to 
day,  but  has  his  own  habit  of  seeing  the  spot  on  the  screen 
disappear.  Such  individual  differences  are  particularly 
marked  when  comparing  lights  differing  in  color. 

In  comparing,  however,  lights  of  approximately  the 
same  intensity  and  color,  as  in  testing  incandescent 
lamps,  there  is  a  convenient  way  of  avoiding  most  of  the 
errors  in  photometry,  which  can  hardly  be  too  strongly 
commended.  It  is  one  of  the  general  processes  of  physi- 
cal investigation,  known  as  the  "  method  of  substitution." 
It  consists  of  comparing  the  standard,  which  we  will  call 
A,  with  an  intermediary  standard  B,  and  then,  leaving 
everything  else  unchanged,  replacing  A  by  the  object  to 
be  tested,  C. 


326  THE   ART   OF   ILLUMINATION. 

In  applying  this  method  to  photometry  proceed  as 
follows :  Place  the  standard  lamp  on  the  rotator  of 
Fig.  123,  and  the  intermediary  standard  in  the  socket 
at  the  other  end  of  the  photometer  bar,  setting  the  sight 
box  at  the  midway  point.  Then  vary  the  intermediary, 
either  by  turning  it  slightly  or  by  shifting  the  rheostat 
belonging  to  it,  until  an  accurate  balance  is  obtained. 
Then  any  lamp  of  equal  intensity  with  the  standard  on  the 
rotator  may  replace  it  without  changing  the  balance. 
This  eliminates  all  the  errors  of  comparison  save  two :  first, 
that  due  to  possible  variation  of  resistance  in  the  rotator, 
and,  second,  that  due  to  a  possible  variation  in  the  ob- 
server's habit  of  seeing  during  the  progress  of  subsequent 
observations. 

Most  standard  lamps  are  intended  to  be  used  in  a  fixed 
azimuth,  and  not  in  rotation,  so  that  the  former  error  may 
enter  unless  the  rotator  is  in  first-class  order.  The  ex- 
istence of  this  error  is  a  strong  argument  in  favor  of 
measuring  lamps  in  one  or  more  fixed  azimuths. 

As  to  the  second  error,  it  is  seldom  of  much  moment  in 
the  case  of  a  practiced  observer,  but  may  be  detected  and 
approximately  evaluated  by  repeating  at  the  close  of  the 
observations  the  original  observation  with  the  funda- 
mental standard,  setting  in  this  case  the  sight  box  without 
varying  either  lamp.  If  the  observer  has  been  uniform  in 
his  habit  of  setting,  and  the  resistance  in  the  rotator  has 
not  varied  materially,  the  sight  box  will  give  balance  at 
the  same  point  as  before. 

The  possible  residual  error  is  that  due  to  the  varying 
resistance  of  the  rotator  when  at  rest  from  its  resistance 
when  in  motion.  This  error  may  be  detected,  if  it  exists, 
by  measuring  the  mean  horizontal  candle-power  of  a  lamp 
having  quite  uniform  horizontal  distribution,  first,  by 


STANDARDS  OF  LIGHT.  327 

rotating,  and,  second,  by  averaging  the  readings  taken, 
say,  in  six  azimuths  60  degrees  apart.  If  the  rotator  has 
introduced  no  error,  the  two  values  thus  obtained  should 
check  each  other  within  the  ordinary  error  of  observation. 
In  incandescent  lamp  testing  there  are  two  general  ways 
of  arranging  the  connections.  In  the  first,  called  the  two- 
circuit  method,  the  working  standard  is  placed  on  an 
independent  source  of  energy,  generally  a  storage  battery, 
brought  to  its  proper  voltage  by  means  of  a  rheostat  in 
circuit  with  it  and  a  voltmeter,  and  kept  constant  during 
the  observations  by  occasional  adjustment  of  the  rheostat, 
if  necessary.  The  lamps  being  tested  are  similarly 
treated.  When  a  storage  battery  is  available,  the  method 


Fig.  124. — Photometer  Circuits. 

is  a  very  satisfactory  one,  the  only  errors  involved  being 
those  in  the  voltmeters,  which  need  to  be  frequently  com- 
pared, and  very  carefully  read. 

The  second  or  single-circuit  method  is  shown  in  dia- 
gram in  Fig.  124.  Here  the  two  lamps  to  be  compared 
are  put  in  multiple  off  the  same  set  of  mains  worked  at  the 
usual  voltage.  The  standard,  B,  is  brought  to  its  proper 
voltage  by  means  of  the  rheostat  and  a  voltmeter,  and 
afterwards  the  voltages  at  the  two  lamps  vary  together, 
if  at  all.  This  method  of  connection  is  very  much  more 
convenient  than  the  two-circuit  method,  especially  in 
alternating-current  stations.  It  is,  moreover,  sufficiently 
precise  if  carefully  applied.  A  second  rheostat  is  em- 
ployed for  lamp  A,  if  the  voltage  of  the  supply  circuit 
varies  from  the  rated  voltage  of  A. 


328 


THE   ART   OF   ILLUMINATION. 


The  essential  difference  between  the  two  methods  is  that 
in  the  two-circuit  scheme  each  lamp  is  tested  rigorously 
at  its  rated  voltage,  while  in  the  single-circuit  method  the 
two  lamps  are  tested  either  at  their  rated  voltages  or  at 
voltages  equally  at  variance  from  these  ratings.  In  the 
latter  case  there  is  a  chance  for  error,  unless  equal  incre- 
ments of  voltage  correspond  to  equal  increments  of  in- 


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Fig.  125. — Variation  of  Light  with  Voltage. 

tensity.  In  other  words,  if  A  and  B  are  once  balanced, 
will  variation  in  the  voltage  of  supply  destroy  that  balance  ? 
In  general  terms,  two  incandescent  lamps  of  the  same 
candle-power  at  some  particular  voltage  will  not  remain 
equal  if  the  voltage  be  changed.  On  the  other  hand,  for 
small  variations  of  voltage  the  difference  will  generally  be 
so  small  as  to  be  within  the  ordinary  errors  of  observation, 
and,  in  fact,  practically  negligible.  Fig.  125  shows  the 
curves  of  variation  of  candle-power  with  voltage,  in  three 
typical  lamps  of  differing  efficiencies.  All  three  show 
an  approximation  to  the  common  rough-and-ready  rule 
of  a  variation  of  one  candle-power  per  volt.  Of  course, 


STANDARDS  OF  LIGHT.  329 

the  slope  of  the  curves  is  the  important  consideration,  and 
this  generally  decreases  slightly  with  the  efficiency  of  the 
lamp. 

A  brief  examination  discloses  the  relations  between  the 
curves.  Suppose  the  working  standard  to  be  a  lamp  of 
moderate  efficiency,  say,  4  watts  per  candle,  as  shown  in 
Curve  B.  Assuming  the  working  voltage  as  100,  a  rise 
of  one  volt  or  a  fall  of  one  volt  increases  or  diminshes  the 
light  by,  as  nearly  as  possible,  .85  candle-power.  If  the 
lamp  under  comparison  corresponds  to  Curve  A,  the 
increment  or  decrement  is  not  far  from  .75  candle-power 
per  volt,  while  with  the  lamp  of  Curve  C  the  change  is  a 
little  over  .9  candle-power  per  volt.  These  three  lamps 
represent  about  as  large  differences  as  will  generally  be 
found,  and  it  is  therefore  safe  to  say  that  for  variations  of 
less  than  one  volt  on  either  side  of  the  normal  the  differ- 
ences in  candle-power  as  between  the  lamps  tested  will  be 
less  than  o.i  candle-power,  and  for  practical  commercial 
testing  may  generally  be  neglected.  But  a  difference  of 
four  or  five  volts  would  obviously  lead  to  variations  of 
a  considerable  fraction  of  a  candle-power,  which  would 
evidently  be  quite  inadmissible. 

The  single-circuit  method  then  must  be  used  with  cau- 
tion, but  when  so  used  is  generally  quite  as  good  as  the 
two-circuit  method,  unless  the  latter  be  applied  with  ex- 
traordinary care.  It  should  be  remembered  that  unless 
the  voltmeters  employed  have  very  open  scales,  the  ordi- 
nary errors  in  reading  them  involve  errors  in  candle- 
power  quite  as  great  as  those  between  two  lamps  on  the 
same  circuit  under  a  slightly  shifting  voltage. 

Of  the  two  methods  the  writer,  on  the  whole,  prefers 
the  single  circuit  one  for  ordinary  use.  It  is  usually  easy 
to  find  a  time  for  testing  when  the  variations  in  voltage 


330  THE   ART   OF   ILLUMINATION. 

are  small  and  slow  enough  to  be  easily  reduced  by  a  little 
attention  to  the  rheostat. 

It  is  not  advisable  in  commercial  testing  to  attempt  the 
comparison  of  incandescent  lamps  with  standards  of  an- 
other character.  Such  comparisons  depend  for  their  cor- 
rectness on  a  knowledge  of  the  absolute  value  of  the 
voltage — a  knowledge  seldom  very  precise.  They  also 
introduce  the  factor  of  color  difference,  which  is  enor- 
mously troublesome,  even  with  trained  observers  and  the 
full  resources  of  a  well-equipped  laboratory. 

When  the  lights  compared  by  means  of  a  Bunsen  or 
Lummer-Brodhun  screen  differ  considerably  in  color, 
absolute  balance  is  attainable  at  no  one  point  on  the  scale. 
The  same  observer  will  obtain  very  regular  apparent 
values  for  the  comparison,  but  another  observer  is  likely 
to  obtain  a  somewhat  different  set  of  values.  Such  per- 
sonal differences  may  easily  amount  to  5  per  cent,  or  more, 
in  comparing,  for  instance,  a  Welsbach  with  a  Hefner 
lamp,  or  an  incandescent  lamp  with  an  enclosed  arc. 

There  will  also  be  considerable  differences  in  the  results 
with  a  single  observer  if  the  absolute  brightness  of  the 
colored  radiants  changes,  even  when  the  relative  bright- 
ness remains  the  same.  That  is,  if  one  were  comparing 
a  Welsbach  and  a  Hefner  lamp,  and  obtained  what  ap- 
peared to  be  a  satisfactory  balance,  that  balance  would  be 
destroyed  by  doubling  the  distance  of  each  light  from  the 
screen. 

These  color  difficulties  are  physiological  and  subjective. 
They  depend  upon  a  property  of  vision  sometimes  known 
as  Purkinje's  law,  stated  by  Von  Helmholtz  as  follows: 
"  Intensity  of  sensation  is  a  function  of  the  luminous  in- 
tensity which  differs  with  the  kind  of  light." 

This  difficulty  in  color  photometry  is  precisely  akin  to 


STANDARDS  OF  LIGHT.  33' 

that  involved  in  comparing  the  loudness  of  two  noises  of 
differing  quality,  although  fortunately  somewhat  less 
serious.  For  example,  one  would  have  extreme  difficulty 
in  forming  any  notion  of  the  relative  loudness  of  a  bugle 
note  and  a  pistol  shot,  or  a  shout  and  a  steam  whistle. 
One's  first  instinctive  effort  at  comparison  would  probably 
be  made  by  investigating  the  distance  at  which  each  sound 
became  inaudible,  or  barely  audible. 

A  similar  procedure  based  on  visual  acuteness  has  often 
been  tried  in  rough  color  photometry.  In  its  crudest 
form  it  consists  of  noting  the  distance  from  each  radiant 
at  which  a  printed  page  held  at  arm's-length  just  becomes 
legible.  A  very  little  experience  will  convince  the  experi- 
menter that  the  results  depend  upon  the  general  state  of 
the  eye,  the  personal  equation  of  the  observer,  practice, 
preconceived  notions  of  the  relative  intensities,  and  other 
factors  so  variable  that  the  result  is  little  better  than 
guesswork.  Yet  this  wildly  inaccurate  method  has  not 
infrequently  been  used  in  estimates  of  street  lighting. 

With  proper  apparatus  and  a  careful  and  unprejudiced 
observer,  however,  the  principle  involved  is  capable  of 
giving  useful  approximate  determinations  of  illumination. 
An  instrument  for  this  purpose  which  has  become  fairly 
well  known  in  this  country  is  Houston  and  Kennelly's 
illuminometer,  shown  in  section  in  Fig.  126.  In  the  cut, 
X,  X  is  a  small  box  thoroughly  blackened  on  the  inside  and 
provided  with  an  eye  tube  T,  T,  pointing  directly  at  a  re- 
movable inclined  block  B,  on  the  face  of  which  is  placed 
a  group  of  printed  test  characters.  A  focusing  eye-piece 
E  enables  any  observer  to  see  the  test  object  distinctly. 
In  the  top  of  the  box  is  a  window  W,  closed  by  a  translu- 
cent diaphragm  of  porcelain,  opal  glass,  or  the  like,  which 
serves  to  illuminate  the  test  object.  This  window  can  be 


332 


THE   ART   OF   ILLUMINATION. 


closed  by  an  opaque  shutter  S,  moved  by  a  rack  and 
pinion,  the  latter  turned  by  a  milled  head  outside  the  box. 

The  instrument  is  used  by  facing  the  window  toward 
the  source  of  illumination,  and  opening  or  closing  the 
shutter  until  the  test  characters  are  just  legible.  A  scale 
attached  to  the  shutter  then  gives  the  illumination  directly 
in  bougie-metres. 

The  scale  is  calibrated  empirically  by  testing  with  a 
source  of  light  of  known  intensity  at  definite  distances. 


Fig.  127.—  Illuminometer. 

The  instrument  is  small  enough  for  the  pocket,  and  is 
very  convenient  for  relative  measurements.  So  far  as 
absolute  values  of  the  illumination  are  concerned,  it  can 
hardly  be  considered  seriously,  unless  in  experienced 
hands,  and  calibrated  by  the  user;  but  in  comparative 
measurements  the  average  error  of  a  single  careful  read- 
ing is  less  than  10  per  cent.,  which  is  a  great  improvement 
on  guesswork.  A  skillful  observer  by  frequently  check- 
ing the  calibration  of  his  instrument  could  bring  the 
absolute  errors  somewhere  nearly  down  to  this  figure. 
The  question  of  color  is  partially  eliminated  by  the  great 
reduction  in  intensity  of  the  light.  As  has  been  noted 
in  a  previous  chapter ,  color  differences  are  inconspicuous 
in  very  faint  light. 


STANDARDS  OF  LIGHT.  333 

To  return  to  photometry  proper,  this  same  expedient 
of  reducing  the  intensity  of  the  light  that  reaches  the  eye 
from  the  photometer  screen  is  of  material  assistance  in 
comparing  colored  lights.  Observing  the  screen  with 
nearly  closed  eyes  makes  comparisons  very  much  easier, 
and  leads  to  fairly  consistent  results.  But  color  percep- 
tion changes  so  notably  in  dim  illumination  that  results 
so  obtained  do  not  represent  working  conditions  nearly 
enough  to  justify  making  any  pretense  of  precision. 

Numerous  expedients  to  avoid  these  difficulties  have 
been  devised,  all  amounting  in  the  last  resort  to  the  selec- 
tion of  conventional  conditions,  representing  the  practical 
requirements  of  illumination.  None  of  them  are  perfectly 
satisfactory  under  all  conditions,  but  probably  the  best 
available  method  is  Crova's.  This  is  based  on  the  experi- 
mental fact  that  in  comparing  two  lights,  even  of  very 
different  color,  their  total  intensities  are  sensibly  propor- 
tional to  their  relative  intensities  in  the  region  of  the 
spectrum  of  wave  length,  about  0.582  M>  that  is,  in  the 
clear  yellow  of  the  spectrum. 

The  troublesome  part  of  such  a  comparison  is  to  segre- 
gate the  rays  of  about  this  wave  length  without  resorting 
to  spectro-photometry,  which  necessitates  the  formation 
of  two  spectra  from  the  two  sources  side  by  side.  Crova 
found  that  a  solution  of  22.3  grams  anhydrous  perchlo- 
ride  of  iron  and  27.2  grams  chloride  of  nickel  in  100  cubic 
centimeters  of  distilled  water  forms  an  absorbing  screen 
that  serves  the  purpose.  The  former  constituent  cuts  out 
the  green  and  blue,  the  latter  the  red.  A  layer  of  this 
standard  solution  7  millimeters  thick,  used  as  a  screen 
through  which  to  observe  the  photometer  screen,  serves 
the  purpose,  although  a  thicker  layer  limits  the  desired 
region  more  closely. 


334  THE   ART    OF   ILLUMINATION. 

The  objection  to  the  method  is  principally  the  large 
amount  of  light  cut  off  by  the  screen,  so  that  it  works  best 
in  comparing  rather  powerful  lights. 

As  a  matter  of  general  practice  such  refinements  are 
seldom  used.  Excepting  arc  lamps,  the  ordinary  sources 
of  light  can  be  compared  without  serious  difficulty  from 
differences  of  color.  With  flame  radiants  a  well  stand- 
ardized Methven  screen  forms  by  far  the  best  working 
standard,  while  in  comparing  incandescent  lamps  the 
working  standard  should  be  an  incandescent  of  moderate 
efficiency. 

In  comparing  arc  lamps  serious  trouble  is  encountered. 
In  the  first  place,  the  difference  between  the  intensity  of 
an  arc  and  any  feasible  standard  is  inconveniently  great, 
and  in  the  second  place  the  colors  are  widely  different, 
especially  in  dealing  with  enclosed  arcs.  The  first  diffi- 
culty may  be  averted  by  using  the  arc  at  a  sufficient  dis- 
tance from  the  screen  to  give  a  proper  working  distance, 
say,  three  or  four  feet,  between  the  screen  and  the  stand- 
ard. In  the  tests  by  the  committee  of  the  National 
Electric  Light  Association  already  quoted,  the  color 
trouble  was  dealt  with  by  observing  the  screen  through 
a  rapidly  rotating  disk  having  narrow  radial  slits.  This 
in  effect  cut  down  the  brilliancy  of  the  screen  to  a  point 
where  color  perception  was  considerably  weakened.  It 
is  rather  doubtful  whether  this  procedure  affected  the 
standard  and  the  arc  in  equal  ratios. 

In  arc  photometry  still  another  troublesome  factor  is 
met,  in  the  tendency  of  the  arc  to  wander  from  side  to 
side  of  the  carbon,  or  to  slowly  rotate,  so  that  the  real 
luminous  intensity  is  very  difficult  to  catch.  In  the  re- 
search just  mentioned  this  was  escaped  by  using  a  pair  of 
mirrors  simultaneously  reflecting  light  from  two  sides  of 


STANDARDS  OF  LIGHT.  335 

the  arc  lamp,  diametrically  opposite,  upon  the  photometer 
screen,  the  direct  radiation  being  screened  off.  The  line 
joining  these  mirrors  was,  of  course,  perpendicular  to  the 
line  of  the  photometer  bar,  and  the  absorption  of  the 
mirror  surfaces  could  readily  be  allowed  for. 

There  is  at  present  no  conventional  method  of  compar- 
ing the  brilliancy  of  different  sources  of  light.  Flames 
are  universally  rated  by  their  intensity  as  measured  in  a 
horizontal  plane,  in  a  direction  generally  45  degrees  from 
the  plane  of  the  flame,  if  the  flame  is  flat,  or  irrespective 
of  direction  in  Argand  and  other  symmetrical  round 
burners,  including  mantle  burners. 

In  the  early  days  of  electric  lighting  the  photometric 
question  assumed  some  importance,  and  all  sorts  of  wild 
statements  were  afloat  as  to  the  power  of  the  new  illumi- 
nant.  Arc  lamps  were  apparently  rated  at  their  momen- 
tary maximum  intensity  on  the  most  favorable  direction. 
The  rivalry  between  makers  of  arc  lamps  did  not  tend  to 
depreciation  of  their  intensity,  and  so  it  came  about  that 
an  open  arc  taking  about  450  watts  was  rated  at  2000 
candle-power,  while  a  similar  arc  of  about  325  watts  was 
rated  at  1200  candle-power. 

While  it  is  possible  that  some  experimenter  at  an 
especially  favorable  moment  may  have  obtained  these 
intensities  in  a  single  direction,  it  is  certain  that  the 
ratings  were  very  soon  regarded  as  merely  conventional. 
They  have  long  since  been  relegated  to  the  category  of 
merely  commercial  designations,  the  rating  bearing  no 
more  precise  relation  to  the  thing  than  does  the  term 
"  best,"  as  applied  to  flour  or  other  commodities. 

When  an  individual  or  a  municipality  contracts  for  a 
2OOO-cp  arc  light,  the  thing  bought,  received,  and  paid 
for  is  an  arc  light  taking  about  450  watts  of  electrical 


336  THE   ART    OF   ILLUMINATION. 

energy,  and  such  is  the  general  understanding  of  the 
term  as  interpreted  at  various  times  by  the  courts.  There 
is  not,  nor  has  there  ever  been,  in  commercial  use  in  this 
country  or  elsewhere  an  arc  lighting  system  using  lamps 
actually  giving  anywhere  near  2000  candle-power,  either 
as  maximum  zonal  intensity  or  as  mean  spherical  intensity. 
The  former  requirement  would  demand  about  750  watts 
at  the  arc,  the  latter  nearly  1200.  Lamps  of  such  power 
have  only  been  used  for  searchlights  and  similar  purposes, 
and  are  far  too  powerful  to  be  advantageously  used  for 
ordinary  illumination. 

In  incandescent  lighting  the  ratings  are  intended  to 
express  the  real  candle-power  of  the  lamps.  Sixteen 
candle-power  is  a  figure  borrowed  from  the  legal  require- 
ments for  gas,  and  corresponded  originally  to  a  measure- 
ment corresponding  to  that  applied  to  gas  flames,  i.  e., 
in  a  horizontal  plane  45  degrees  from  the  plane  of  the 
curve  formed  by  the  filament. 

With  the  introduction  of  looped  and  spiraled  filaments 
giving  a  better  distribution  of  light  than  the  simple  U- 
shaped  filament,  demand  arose  for  a  method  of  measure- 
ment which  would  credit  these  lamps  with  their  just  due.. 
Hence  arose  the  measurement  of  mean  horizontal  candle- 
power  by  rotating  the  lamp.  This  credits  the  lamp  with 
its  just  horizontal  candle-power  as  against  a  lamp  giving 
1 6  candle-power  only  in  certain  horizontal  direction,  but 
it  fails  to  give  credit  for  gains  in  spherical  distribution, 
and  puts  a  premium  on  lamps  with  long  Li-filaments 
adapted  to  throw  out  a  large  proportion  of  horizontal 
illumination. 

Mean  spherical  candle-power,  i.  e.,  total  luminous  flux, 
is  unquestionably  the  fairest  basis  of  comparison  between 
various  sources  of  light,  but  it  is  somewhat  troublesome 


STANDARDS  OF  LIGB  337 

to  measure,  and  runs  counter  to  long-  established  custom 
and  legal  requirements  as  to  gas  lighting.  It  is  certainly 
desirable  that  a  uniform  method  should  be  established  for 
all  radiants,  and  this  is  no  easy  matter.  There  is  a  strong 
tendency  to  apply  the  mean  spherical  measurements  to  arc 
lamps,  although  the  lower  hemispherical  candle-power 
is  sometimes  used  instead,  on  the  ground  that  downward 
light  is  the  proper  criterion  of  useful  illumination.  This 
rating  is  approximately  true  of  lamps  having  reflectors 
over  them,  but  it  is  certainly  not  true  in  general,  for  it 
neglects  the  very  great  effectiveness  of  diffuse  reflection 
from  walls  and  ceiling. 

The  fact  is  that  no  simple  rating  can  be  applied  with 
equal  fairness  to  all  commercial  sources  of  light,  by  reason 
of  their  very  great  diversity  in  the  nature  of  the  light- 
distribution. 

The  mean  spherical  measurement  comes  nearer  to 
general  fairness  than  any  other,  and  could  it  be  uni- 
versally adopted  it  would  afford  a  very  satisfactory  basis 
of  comparison.  As  a  practical  standard  at  the  present 
time,  it  leaves  considerable  to  be  desired. 

Mean  horizontal  candle-power  is  by  far  the  easiest  thing 
to  measure,  and  it  is  to  be  recommended,  save  in  the  com- 
parison of  radiants  deliberately  planned,  as  in  case  of 
intensive  gas  burners,  the  American  type  of  Nernst  lamp, 
and  certain  arcs  and  incandescents,  to  give  particularly 
strong  illumination  in  some  other  direction. 

The  thing  most  to  be  desired  in  practical  photometric 
work  is  a  general  international  convention  defining  em- 
pirically, if  need  be,  certain  bases  of  work.  A  working 
reproducible  standard  of  greater  intensity  and  better  color 
than  the  Carcel  or  Hefner  lamp  is  badly  needed.  As 
actual  standards  for  use  on  the  photometer  bar,  nothing 


338  THE   ART   OF   ILLUMINATION. 

can  be  better  than  incandescent  lamps,  but  as  has  already 
been  noted,  they  are  not  reproducible.  The  nearest  ap- 
proach to  a  reproducible  standard  of  good  size  and  color 
at  present  available  seems  to  be  the  Vernon-Harcourt 
lo-cp  pentane  lamp,  which  is  the  present  official  standard 
in  London.  It  has  not  been  subjected  to  as  searching 
and  protracted  an  investigation  as  the  Hefner  lamp,  but 
the  reports  so  far  obtained  from  it  are  highly  encourag- 
ing, while  its  intensity  and  color  are  great  advantages  in 
passing  from  it  to  the  more  powerful  modern  radiants. 

Granted  a  proper  standard,  there  is  also  needed  a 
definite  conventional  method  of  dealing  with  the  color 
difficulty.  This  involves  a  tougher  problem  even  than 
the  standard  itself.  Possibly  Crova's  method,  or  some 
modification  of  it,  might  be  made  to  serve  a  useful  pur- 
pose. Finally,  aside  from  the  difficulty  of  comparing 
lights  differing  widely  in  color,  there  remains  the  question 
of  the  different  illuminative  values  of  such  lights  when  put 
into  practical  service..  This  again  suggests  the  question 
of  measuring  illumination,  instead  of  the  intensity  of  the 
radiants,  but  as  has  already  been  indicated  there  are  no 
methods  of  measuring  illumination  comparable  in  pre- 
cision with  ordinary  photometry,  which  is  saying  little 
enough. 

It  is  to  be  hoped  that  the  recently  organized  Bureau  of 
Standards  may  facilitate  the  study  of  these  puzzling  mat- 
ters, and  promote  an  international  photometric  congress 
that  can  give  general  sanction  to  a  definite  programme 
in  commercial  photometry. 

A  great  deal  of  time  and  effort  has  been  wasted  in  this 
world  in  the  promulgation  of  so-called  "  absolute  "  stand- 
ards, referred  in  a  perfectly  definite  way  to  immutable 
constants  of  nature.  Desirable  as  they  are,  it  is  of  far 


STANDARDS  OF  LIGHT.  339 

greater  importance  to  have  a  convenient,  reproducible,  and 
international  set  of  units  in  universal  use.  The  metric 
system  started  on  its  career  as  an  absolute  system,  but  its 
value  lies  not  in  the  supposed  relation  of  its  units  to 
natural  constants,  but  in  their  relation  to  each  other,  and 
in  its  well-nigh  universal  acceptance  as  the  basis  of 
scientific  measurements  of  length  and  mass. 

Standards  as  concrete  things  may  be  constantly  suscep- 
tible of  improvement  without  limit.  They  are  important 
practically  only  in  proportion  to  their  general  recognition 
at  a  certain  conventional  determinable  value. 


INDEX. 


Absorption,  selective,  28 

Acetylene,  75 

,  burners  for,  79 

,'  generators  for,  So 

,  dangers  of,  77 

,  preparation  of,  76 

,  value  of,  8 1 

After-images,  10 

Air-gas  66 

,  cost  of,  67 

— • machines,  66 

Architectural  illumination,  283 

illumination,  funda- 
mental principles  of,  284 

Arc,  best  length  of,  143 

,  relation  between  length 

of,  and  voltage,  143 

,  relation  between  cur- 
rent density  and  light  in,  142 

.  .relation   between   cur- 


rent and  efficiency  in,  160 
Arcs,  actual  intensities  of,  248 

,  alternating  and  direct 

current  comparison  of,  158 
,    alternating,   best    fre- 
quency for,  157 
.    alternating,    distribu- 


tion of  light  from,  156 
,  alternating,  advantages 

of,  155 

-,  alternating,  annual  sav- 


ing from,  260 

,  alternating  current,  154 

,      alternating      current 

series,  259 

,  enclosed,  144 

,  low  voltage,  144 

,  classification  of,  248 

computing     illumina- 


tion from,  251 

,      distribution      curves 

from  various,  249 

,    distribution    of    light 

from,  148 
— — ,  enclosed,  amperage  of, 


150 


Arcs,     enclosed,     distribution 
of  light  from,  150 

,     enclosed,      character- 
istics of,  147 

,  enclosed,  consumption 

of  carbon  in,  145 

enclosed,    voltage    in, 


146 

,      illumination 

from,  255 
,     for      lighting 


large 


rooms,  217 

alternating,  objections 


to,  155 

,  rating  of,  335 

result   of    distribution 


from,  248 

Bougie  d ecu n ale,  317 
Bougie-meter,  5 
Bracket  fixtures,  271 
Bulbs,  exhaustion  of,  100 

,  Malignani   process  for 

exhausting,  100 
Bunsen    screen,     construction 

of,  318 
Burner,  Argand,  71 

bat's-wing,  71 

fishtail,  71 

oxy-hydrogen,  83 

Siemens,  74 

Welsbach,  85 

Welsbach,  form  of,  86 

Wenham,  73 


Burners,  regenerative,  73 
Burning  fluids,  59 

Calcic  carbide,  76 

carbide,  cost  of,  81 

Candle-foot,  5 

Candle  power,  mean  spherical, 
105 

,  standard,  314 

Candles,  62 

,  efficiency  of,  62 

Carcel  lamp,  314 


341 


342 


INDEX. 


Ceiling  lighting,  advantage  of, 

232      ' 
lights  in  halls,  216 

lights,   practical  effect 

of,  196 

Churches,  amount  of  light  for, 
225 

,  choice  of  lights  for,  224 

,  illumination  of,  224 

Coal  gas,  68 

gas,  composition  of,  68 

gas  ,  impurities  in,  69 

Color,   effects  of  dilution  on, 

44 

,  fundamental  law  of,  23 

,  in  illumination,  23 

,  of  walls  in  illumination, 

55 

Color-blindness,  effect  of,  30 
Color-photometry,  330 

Crova's  method  of,  333 

Colors,  changeable,  26 

,  from  pigments,  25 

in  very  dim  light,  29 

,  luminosity  of,  32 

,  matching,  29 

Colored    illumination,    limita- 
tions of,  289 

—  light  on  fabrics,  34 

light,  effect  of,  33 

Common  illuminants,  cost   of, 

94, 

illuminants,   properties 

of,  93 

Crater,  temperature  of,  142 
Cross-suspensions,  273 

Danger  from  light  oils,  92 
Daylight,  intensity  of,  21 
Decorative    lighting  of    large 

buildings,  280 
—  lighting,  temporary,  289 

lighting,       miniature 

lamps  for,  291 

Diffuse  lighting,  development 
of,  309 

• lighting,  objections  to, 

310 

Diffusion,  difficulty   of  check- 
ing, 54 

,  help  received  from,  188 

in  large  rooms,  211 

,  relation  of,  to  quantity 

of  light,  189 


Display  and  scenic  illumina- 
tion, 275 

Domestic  lighting,  illuminants 
for,  183 

lighting,  importance  of 

low  intrinsic  brilliancy  in, 
184 

illumination,    8     c.  p. 

lamps  in,  210 

illumination,     mantle 

burners  in,  210 
lighting,    quantity    of 

light  for,  190 
Draughting     rooms,    inverted 

arcs  in,  236 

rooms,   light    required 

in,  235 

Electric  arc,  140 

arc,  crater  of,  141 

Enclosed  arcs,  annual  saving 
from,  259 

• arcs,  illumination  from, 

258 

Exposition  buildings,  illumina- 
tion of,  242 

lighting,  principles  of, 

285 

Eye,  properties  of,  2 

Factories,  illumination  re- 
quired for,  240 

Fats,  58 

Fechner's  law,  4 

Filament  of  Auer  von  Wels- 
bach,  124 

Filaments  of  refractory  mate- 
rial, 123 

,  disintegration  of,  no 

,  flashing,  98 

.  effect  of  flashing  on,  99 

,  manufacture  of,  96 

,  material  of,  96 

,  practical  dimensions  of, 

114 

,  section  of,  101 

,  shapes  of,  101 

from  soluble   cellulose, 


96 
Firefly,  efficiency  of  light  of, 

138 
,      emission      spectrum 

from,  137,  138 
,  light  of,  136 


INDEX. 


343 


Fire  risks  of  illumination,  295 
Flames,  luminous,  56 
Fraunhofer  lines,  24 

Gas-burners,  71 

Hefner  unit,  315 
Holophane  globes,  175 

globes,  classes  of,  177 

globes,    distribution  of 

light  from,  180 

- —  globes,  structure  of,  175 

globes,  weak  points  of, 


179 

Illuminants,  choice  of,  193 

,  colors  of,  36 

,  color  properties  of,  27 

,  conception  of  efficiency 

in,  303 

,  incandescent,  56 

,  ultimate   efficiency  of, 

304 
Illumination,  apparent,  12 

,  common  fault  in,  296 

,  computation  of,  for  a 

room,  191 

,  best  direction  of,  17 

,  effect  of  direction  in,  3 

,  effective,  186 

,  effect  of  height  on,  223 

,  formulae  for  computing, 

194 

,  general,  2 

of  a  hall,  computing,  213 

of  very  high  rooms,  220 

of  a  modern  house,  202 

,  predominant   direction 

of,  217 

-,  needed    improvements 


m.  304 

over-brilliant,  301 
relation   of,   to   yellow 
component,  33 
scenic,  i 
standards  of,  5 
necessary  strength  of, 
18 

Illuminometer,  331 
Interior    illumination,    limita- 
tions imposed  upon,  195 
Incandescent   lamps,    rotators 

for,  323 
lamps,  value  of,  103 


Incandescent  electric  light.  95 
Incandescents,   color    of  light 

in,  113 

,  actual  cost  of,  117 

,   effect  of    temperature 

on  life  of,  116 

,  high  efficiency,  no 

importance  of  sorting, 


intrinsic  brilliancy  of, 


120 


in 


,      illumination      curves 

from,  262 

,  light-curves  from,  104 

,  life  of,  115,  116,  117 

life  of,  in  candle-hours, 


118 

,  low  efficiency,  113 

,   nominal  candle-power 

of,  104 

,  position  of  axis  in,  108 

,  rated  efficiency  of,  109 

,  real  efficiency  of,  122 

,  rating  of,  336 

,  rating,  107 

relation    between    in- 


tensity and  energy  in,  112 

relation   of    light  and 


voltage  in,  in 

,  relation  of  temperature 

and   efficiency  'in,    109,    in, 

112 

-,  need  of  good  regulation 


for,  119 

,  standard  sizes  of,  113 

,  total  light  from,  104 

,  value  of,  118,  119,  120 

variation  of  light,  with 


permis- 


voltage  in,  328 

Inertia,  visual,  13 

Intrinsic     brilliancy 
sible,  307 

— brightness,  8 

brightness,  table  of,  9 

Inverse  squares,  law  of,  6 

Inverted  arcs,  unilateral  light- 
ing by,  239 

Iris,  action  of,  n 

Kerosene,  61 

Lamp,  arc,  fluctuations  of,  14 

,    incandescent    electric, 

13 


344 


INDEX. 


Lamp,  incandescent  flickering 
of,  13 

,  Nernst,  124 

,  Nernst,  advantage  of, 

on  high  voltage,  133 

,  Nernst,  American  form 

of,  129 

,    Nernst,    arrangement 

of,  126 

,  Nernst,   ballast    resist- 
ance in,  127 

,  Nernst,  on  continuous 

current,  132 

,  Nernst,   intrinsic    bril- 
liancy of,  131 

,  Nernst,  life  of,  129,  131 

,      Nernst,      light-curve 

from,  133 

,  Nernst,  tests  of,  130 

-,  Nernst,  variation  of  re- 


sistance in,  124 

,  "  Rochester,"  63 

,  vacuum  tube,  134 

-,  vacuum   tube,  color  of 


light  from,  135 

vacuum   tube,   difficul- 


ties with,  134 
Lamps  and  candles,   function 

of,  194 
,  incandescent,  of   large 

power,  233 

,  kerosene,  63 

,  oil,  63 

,  Roman,  57 

-,  silvered  bulb,  171 


Light,    artificial,    sources     of, 

57 

,  diffused,  value  of,  187 

Lights,  importance  of  steadi- 
ness in,  241 

Lighting,  criterion  of  effective, 
306 

"  Lucigen  "  torch,  64 

Lummer-Brodhun  screen,  322 

Lux,  5 

Mantle  burners,  86 

burners  for  air-gas,  91 

burners,    color  of  light 

of,  89 

burners,  efficiency  of,  88 

burners,  life  of,  89 


Mantles,  composition  of,  85 
Mast-arms,  271 


Mean  spherical  candle  power 
as  a  basis  of  rating,  336 

Methveivscreen,  317 

Miniature  lamps,  objections  to, 
292 

Monuments,  illuminating,  281 

Moonlight  schedule,  265 

Municipal  lighting,  268 

Nernst  filament,  efficiency  of, 
129 

Oils,  combustion  of,  63 

Paraffin,  61 

Petroleum,  60 

,  composition  of,  60 

products,  61 

Phosphorescence,  possible 
value  of,  305 

Photometer  bench,  321 

,  Bunsen,  20,  320 

circuits,  327 

,  daylight,  19 

,  practical  arrangement 

of,  324 

Photometers,  portable,  322 

Photometry  of  arc  lamps,  334 

,  "  method  of  substitu- 
tion " in,  325 

Plane  of  illumination,  190 

Pole-top  fixtures,  270 

Projectors,  stage,  278 

Public  buildings,  lighting,  227 

lights,  relation  of  gen- 
eral illumination  to,  264 

squares,  lighting  of,  264 

Quantity  of  light  required  in 
large  buildings,  212 

Railway  stations,  lighting,  233 
stations,  spacing  of  arcs 

in,  234 

Rare  earths,  properties  of,  86 
Reflection,  38 

,  asymmetric,  42,  49 

,  asymmetric,  in  fabrics, 

50 

,  coefficient  of,  47 

— ,  diffuse,  39 

,  coefficients  of  diffuse,  53 

,  coefficients  of  regular, 

47 


INDEX. 


345 


Reflection,  selective,  effect  of, 

total  intensity  of,  41 

losses  in,  48 

multiple,  45 

diffuse,  nature  of,  40 

regular,  38 

selective,  43 

Reflector  lamps,  171 

lamps,  objections  to,  173 

Reflectors,  163 

for  inverted  arcs,  236 

,  economy  o£,  198 

Rooms,  light  required  to  illu- 
minate, 199 

Search  light,  7 

lights,  297 

lights,  use  of,  298 

Selective  coloration,  effect   of 

material  on,  50 
Shades,  163 

light  intercepted  by,  166 

paper  and  fabric,  165 

reflecting,  167 

reflecting,  tests  of,  169 

requirements  for,  165 

Shadows,  function  of,  16 
"  Shotgun  diagram,"  120 
diagram,    interpreta- 
tion of,  121 

Single-circuit  method,  327 
Snow-blindness,  3 
Spectra  of  colors,  25 
Spectrum,  24 
Standards,    requirements    for, 

3i3 

,   relation  between  pri- 
mary, 318 

,  secondary,  316 

Stearin,  %Q 

Street  lighting,  244 

lighting,  annual  hours 

of,  265, 

lighting,  contracts  for, 

269 
r  lighting,  cost  of,  267 


Street  lighting,  fixtures  for,  270 

lighting,   incandescents 

in,  261 

lights,  location  of,  263 

light,    spacing  of  vari- 
ous, 266 

Streets,    amount  of    light    re- 
quired for,  254 

,    effective    illumination 

in,  257 

-,  computing  illumination 


for,  244 
,  minimum  illlumination 

in,  247 
,  principles  of  lighting, 

246 

Temporary  lighting  effects,  233 
lighting,  installation  of, 

293 
Theaters,  ceiling  lighting  for, 

231 
,   actual   floor  space  of, 

229 

,  illumination  of,  228 

,  light  required  in,  230 

,  location  of  lights  in,  230 

,  stage  lighting  in,  276 

Two-circuit  method,  327 

Vacuum     tube,    efficiency    of, 

135 

Velvet,  action  of  dyes  in,  51 
Vernon      Harcourt      pentane 

standard,  337 
Violle's  unit,  313 
Visual  usefulness,  306 

Walls,  diffusion  from,  197 
Water-gas,  69 

,  composition  of,  70 

,  danger  from,  70 

Waxes,  58 

White  light,  composition  of,  24 

Workshops,  arc  lights  in,  218 

,  light  required  in,  241 

,  mantle  burners  in,  219 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below, 
or  on  the  date  to  which  renewed.  Renewals  only: 

Tel.  No.  642-3405 

Renewals  may  be  made  4  days  prior  to  date  due. 
Renewed  books  are  subject  to  immediate  recall. 


AUGI8 
FLB1 L 1390 


OUN  U  5  1990 


LD21A-40m-8,'71 
(P6572slOH76-A-32 


General  Library 

University  of  California 

Berkeley 


gubjea  Jc 

i 


m 


YC   12901 


GENERAL  LIBRARY  -  U.C.  BERKELEY 


• 


